W3C

HTML 5.2

Editor’s Draft,

6. Loading Web pages

This section describes features that apply most directly to Web browsers. Having said that, except where specified otherwise, the requirements defined in this section do apply to all user agents, whether they are Web browsers or not.

6.1. Browsing contexts

A browsing context is an environment in which Document objects are presented to the user.

A tab or window in a Web browser typically contains a browsing context, as does an iframe or frames in a frameset.

A browsing context has a corresponding WindowProxy object.

A browsing context has a session history, which lists the Document objects that the browsing context has presented, is presenting, or will present. At any time, one Document in each browsing context is designated the active document. A Document's browsing context is that browsing context whose session history contains the Document, if any. (A Document created using an API such as createDocument() has no browsing context.) Each Document in a browsing context is associated with a Window object.

In general, there is a 1-to-1 mapping from the Window object to the Document object. There are two exceptions. First, a Window can be reused for the presentation of a second Document in the same browsing context, such that the mapping is then 1-to-2. This occurs when a browsing context is navigated from the initial about:blank Document to another, with replacement enabled. Second, a Document can end up being reused for several Window objects when the document.open() method is used, such that the mapping is then many-to-1.

A Document does not necessarily have a browsing context associated with it. In particular, data mining tools are likely to never instantiate browsing contexts.


A browsing context can have a creator browsing context, the browsing context that was responsible for its creation. If a browsing context has a parent browsing context, then that is its creator browsing context. Otherwise, if the browsing context has an opener browsing context, then that is its creator browsing context. Otherwise, the browsing context has no creator browsing context.

If a browsing context A has a creator browsing context, then the creator origin, creator URL, and creator base URL are the origin, URL, and base URL, respectively, of the Document that was the active document of that creator browsing context at the time A was created.

To create a new browsing context:

  1. Call the JavaScript InitializeHostDefinedRealm() abstract operation with the following customizations:

    • For the global object, create a new Window object window.

    • For the global this value, create a new WindowProxy object windowProxy, whose [[Window]] internal slot value is window.

      The internal slot value is updated when navigations occur.

    • Let realm execution context be the created JavaScript execution context.

  2. Set the new browsing context’s associated WindowProxy to windowProxy.

  3. Let document be a new Document, whose URL is about:blank, which is marked as being an HTML document, whose character encoding is UTF-8, and which is both ready for post-load tasks and completely loaded immediately.

  4. Set the origin of document:

  5. If the new browsing context has a creator browsing context, then set document’s referrer to the creator URL.

  6. Ensure that document has a single child html node, which itself has two empty child nodes: a head element, and a body element.

  7. Implement the sandboxing for document.

  8. Add document to the new browsing context’s session history.

  9. Set window’s associated Document to document.

  10. Set up a browsing context environment settings object with realm execution context.

6.1.1. Nested browsing contexts

Certain elements (for example, iframe elements) can instantiate further browsing contexts. These are called nested browsing contexts. If a browsing context P has a Document D with an element E that nests another browsing context C inside it, then C is said to be nested through D, and E is said to be the browsing context container of C. If the browsing context container element E is in the Document D, then P is said to be the parent browsing context of C and C is said to be a child browsing context of P. Otherwise, the nested browsing context C has no parent browsing context.

A browsing context A is said to be an ancestor of a browsing context B if there exists a browsing context A' that is a child browsing context of A and that is itself an ancestor of B, or if the browsing context A is the parent browsing context of B.

A browsing context that is not a nested browsing context has no parent browsing context, and is the top-level browsing context of all the browsing contexts for which it is an ancestor browsing context.

The transitive closure of parent browsing contexts for a nested browsing context gives the list of ancestor browsing contexts.

The list of the descendant browsing contexts of a Document d is the (ordered) list returned by the following algorithm:

  1. Let list be an empty list.
  2. For each child browsing context of d that is nested through an element that is in the Document d, in the tree order of the elements nesting those browsing contexts, run these substeps:

    1. Append that child browsing context to the list list.
    2. Append the list of the descendant browsing contexts of the active document of that child browsing context to the list list.
  3. Return the constructed list.

A Document is said to be fully active when it has a browsing context and it is the active document of that browsing context, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the Document through which it is nested is itself fully active.

Because they are nested through an element, child browsing contexts are always tied to a specific Document in their parent browsing context. User agents must not allow the user to interact with child browsing contexts of elements that are in Documents that are not themselves fully active.

A nested browsing context can be put into a delaying load events mode. This is used when it is navigated, to delay the load event of the browsing context container before the new Document is created.

The document family of a browsing context consists of the union of all the Document objects in that browsing context’s session history and the document families of all those Document objects. The document family of a Document object consists of the union of all the document families of the browsing contexts that are nested through the Document object.

window . top

Returns the WindowProxy for the top-level browsing context.

window . parent

Returns the WindowProxy for the parent browsing context.

window . frameElement

Returns the Element for the browsing context container.

Returns null if there isn’t one, and in cross-origin situations.

The top IDL attribute on the Window object of a Document in a browsing context b must return the WindowProxy object of its top-level browsing context (which would be its own WindowProxy object if it was a top-level browsing context itself), if it has one, or its own WindowProxy object otherwise (e.g., if it was a detached nested browsing context).

The parent IDL attribute on the Window object of a Document that has a browsing context b must return the WindowProxy object of the parent browsing context, if there is one (i.e., if b is a child browsing context), or the WindowProxy object of the browsing context b itself, otherwise (i.e., if it is a top-level browsing context or a detached nested browsing context).

The frameElement IDL attribute, on getting, must run the following algorithm:

  1. Let d be the Window object’s associated Document.

  2. Let context be d’s browsing context.

  3. If context is not a nested browsing context, return null and abort these steps.

  4. Let container be context’s browsing context container.

  5. If container’s node document’s origin is not same origin-domain with the entry settings object’s origin, then return null and abort these steps.

  6. Return container.

6.1.2. Auxiliary browsing contexts

It is possible to create new browsing contexts that are related to a top-level browsing context without being nested through an element. Such browsing contexts are called auxiliary browsing contexts. Auxiliary browsing contexts are always top-level browsing contexts.

An auxiliary browsing context has an opener browsing context, which is the browsing context from which the auxiliary browsing context was created.

The opener IDL attribute on the Window object, on getting, must return the WindowProxy object of the browsing context from which the current browsing context was created (its opener browsing context), if there is one, if it is still available, and if the current browsing context has not disowned its opener; otherwise, it must return null.

On setting the opener attribute, if the new value is null then the current browsing context must disown its opener; if the new value is anything else then the user agent must call the [[DefineOwnProperty]] internal method of the Window object, passing the property name "opener" as the property key, and the Property Descriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true } as the property descriptor, where value is the new value.

6.1.3. Security

A browsing context A is familiar with a second browsing context B if one of the following conditions is true:


A browsing context A is allowed to navigate a second browsing context B if the following algorithm terminates positively:

  1. If A is not the same browsing context as B, and A is not one of the ancestor browsing contexts of B, and B is not a top-level browsing context, and A’s active document’s active sandboxing flag set has its sandboxed navigation browsing context flag set, then abort these steps negatively.
  2. Otherwise, if B is a top-level browsing context, and is one of the ancestor browsing contexts of A, and A’s active document’s active sandboxing flag set has its sandboxed top-level navigation browsing context flag set, then abort these steps negatively.
  3. Otherwise, if B is a top-level browsing context, and is neither A nor one of the ancestor browsing contexts of A, and A’s Document's active sandboxing flag set has its sandboxed navigation browsing context flag set, and A is not the one permitted sandboxed navigator of B, then abort these steps negatively.
  4. Otherwise, terminate positively!

An element has a browsing context scope origin if its Document's browsing context is a top-level browsing context or if all of its Document's ancestor browsing contexts all have active documents whose origin are the same origin as the element’s node document’s origin. If an element has a browsing context scope origin, then its value is the origin of the element’s node document.

6.1.4. Groupings of browsing contexts

Each browsing context is defined as having a list of one or more directly reachable browsing contexts. These are:

The transitive closure of all the browsing contexts that are directly reachable browsing contexts forms a unit of related browsing contexts.

Each unit of related browsing contexts is then further divided into the smallest number of groups such that every member of each group has an active document with an origin that, through appropriate manipulation of the document.domain attribute, could be made to be same origin-domain with other members of the group, but could not be made the same as members of any other group. Each such group is a unit of related similar-origin browsing contexts.

There is also at most one event loop per unit of related similar-origin browsing contexts (though several units of related similar-origin browsing contexts can have a shared event loop).

6.1.5. Browsing context names

Browsing contexts can have a browsing context name. By default, a browsing context has no name (its name is not set).

A valid browsing context name is any string with at least one character that does not start with a U+005F LOW LINE character. (Names starting with an underscore are reserved for special keywords.)

A valid browsing context name or keyword is any string that is either a valid browsing context name or that is an ASCII case-insensitive match for one of: _blank, _self, _parent, or _top.

These values have different meanings based on whether the page is sandboxed or not, as summarized in the following (non-normative) table. In this table, "current" means the browsing context that the link or script is in, "parent" means the parent browsing context of the one the link or script is in, "top" means the top-level browsing context of the one the link or script is in, "new" means a new top-level browsing context or auxiliary browsing context is to be created, subject to various user preferences and user agent policies, "none" means that nothing will happen, and "maybe new" means the same as "new" if the "allow-popups" keyword is also specified on the sandbox attribute (or if the user overrode the sandboxing), and the same as "none" otherwise.

Keyword Ordinary effect Effect in an iframe with...
sandbox="" sandbox="allow-top-navigation"
none specified, for links and form submissions current current current
empty string current current current
_blank new maybe new maybe new
_self current current current
_parent if there isn’t a parent current current current
_parent if parent is also top parent/top none parent/top
_parent if there is one and it’s not top parent none none
_top if top is current current current current
_top if top is not current top none top
name that doesn’t exist new maybe new maybe new
name that exists and is a descendant specified descendant specified descendant specified descendant
name that exists and is current current current current
name that exists and is an ancestor that is top specified ancestor none specified ancestor/top
name that exists and is an ancestor that is not top specified ancestor none none
other name that exists with common top specified none none
name that exists with different top, if familiar and one permitted sandboxed navigator specified specified specified
name that exists with different top, if familiar but not one permitted sandboxed navigator specified none none
name that exists with different top, not familiar new maybe new maybe new

Most of the restrictions on sandboxed browsing contexts are applied by other algorithms, e.g., the navigation algorithm, not the rules for choosing a browsing context given a browsing context name given below.


An algorithm is allowed to show a popup if any of the following conditions is true:

  • The task in which the algorithm is running is currently processing an activation behavior whose click event was trusted.
  • The task in which the algorithm is running is currently running the event listener for a trusted event whose type is in the following list:

    • change
    • click
    • dblclick
    • mouseup
    • reset
    • submit
  • The task in which the algorithm is running was queued by an algorithm that was allowed to show a popup, and the chain of such algorithms started within a user-agent defined timeframe.

    For example, if a user clicked a button, it might be acceptable for a popup to result from that after 4 seconds, but it would likely not be acceptable for a popup to result from that after 4 hours.


The rules for choosing a browsing context given a browsing context name are as follows. The rules assume that they are being applied in the context of a browsing context, as part of the execution of a task.

  1. If the given browsing context name is the empty string or _self, then the chosen browsing context must be the current one.
  2. If the given browsing context name is _parent, then the chosen browsing context must be the parent browsing context of the current one, unless there isn’t one, in which case the chosen browsing context must be the current browsing context.
  3. If the given browsing context name is _top, then the chosen browsing context must be the top-level browsing context of the current one, if there is one, or else the current browsing context.
  4. If the given browsing context name is not _blank and there exists a browsing context whose name is the same as the given browsing context name, and the current browsing context is familiar with that browsing context, and the user agent determines that the two browsing contexts are related enough that it is ok if they reach each other, then that browsing context must be the chosen one. If there are multiple matching browsing contexts, the user agent should select one in some arbitrary consistent manner, such as the most recently opened, most recently focused, or more closely related.
  5. Otherwise, a new browsing context is being requested, and what happens depends on the user agent’s configuration and abilities — it is determined by the rules given for the first applicable option from the following list:
    If the algorithm is not allowed to show a popup and the user agent has been configured to not show popups (i.e., the user agent has a "popup blocker" enabled)

    There is no chosen browsing context. The user agent may inform the user that a popup has been blocked.

    If the current browsing context’s active document’s active sandboxing flag set has the sandboxed auxiliary navigation browsing context flag set.

    Typically, there is no chosen browsing context.

    The user agent may offer to create a new top-level browsing context or reuse an existing top-level browsing context. If the user picks one of those options, then the designated browsing context must be the chosen one (the browsing context’s name isn’t set to the given browsing context name). The default behavior (if the user agent doesn’t offer the option to the user, or if the user declines to allow a browsing context to be used) must be that there must not be a chosen browsing context.

    If this case occurs, it means that an author has explicitly sandboxed the document that is trying to open a link.

    If the user agent has been configured such that in this instance it will create a new browsing context, and the browsing context is being requested as part of following a hyperlink whose link types include the noreferrer keyword
    A new top-level browsing context must be created. If the given browsing context name is not _blank, then the new top-level browsing context’s name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context. The creation of such a browsing context is a new start for session storage.

    If it is immediately navigated, then the navigation will be done with replacement enabled.

    If the user agent has been configured such that in this instance it will create a new browsing context, and the noreferrer keyword doesn’t apply
    A new auxiliary browsing context must be created, with the opener browsing context being the current one. If the given browsing context name is not _blank, then the new auxiliary browsing context’s name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context.

    If it is immediately navigated, then the navigation will be done with replacement enabled.

    If the user agent has been configured such that in this instance it will reuse the current browsing context
    The chosen browsing context is the current browsing context.
    If the user agent has been configured such that in this instance it will not find a browsing context
    There must not be a chosen browsing context.

    User agent implementors are encouraged to provide a way for users to configure the user agent to always reuse the current browsing context.

    If the chosen browsing context picked above, if any, is a new browsing context, then:

    1. Let flagSet be the current browsing context’s active document’s active sandboxing flag set.

    2. If flagSet’s sandboxed navigation browsing context flag is set, then the current browsing context must be set as the new browsing context’s one permitted sandboxed navigator.

    3. If flagSet’s sandbox propagates to auxiliary browsing contexts flag is set, then all the flags that are set in flagSet must be set in the new browsing context’s popup sandboxing flag set.

6.1.6. Script settings for browsing contexts

When the user agent is required to set up a browsing context environment settings object, given a JavaScript execution context execution context, it must run the following steps:

  1. Let realm be the value of execution context’s Realm component.

  2. Let window be realm’s global object.

  3. Let url be a copy of the URL of the Document with which window is associated.

  4. Let settings object be a new environment settings object whose algorithms are defined as follows:

    The realm execution context

    Return execution context.

    The module map

    Return the module map of the Document with which window is currently associated.

    The responsible browsing context

    Return the browsing context with which window is associated.

    The responsible event loop

    Return the event loop that is associated with the unit of related similar-origin browsing contexts to which window’s browsing context belongs.

    The responsible document

    Return the Document with which window is currently associated.

    The API URL character encoding

    Return the current character encoding of the Document with which window is currently associated.

    The API base URL

    Return the current base URL of the Document with which window is currently associated.

    The origin

    Return the origin of the Document with which window is currently associated.

    The creation URL

    Return url.

    The HTTPS state

    Return the HTTPS state of the Document with which window is currently associated.

  5. Set realm’s [[HostDefined]] field to settings object.

  6. Return settings object.

6.2. Security infrastructure for Window, WindowProxy, and Location objects

Although typically objects cannot be accessed across origins, the web platform would not be true to itself if it did not have some legacy exceptions to that rule that the web depends upon.

6.2.1. Integration with IDL

When perform a security check is invoked, with a platformObject, realm, identifier, and type, run these steps:

  1. If platformObject is a Window or Location object, then:

    1. Repeat for each e that is an element of CrossOriginProperties(platformObject):

      1. If SameValue(e.[[Property]], identifier) is true, then:

        1. If type is "method" and e has neither [[NeedsGet]] nor [[NeedsSet]], then return.

        2. Otherwise, if type is "getter" and e.[[NeedsGet]] is true, then return.

        3. Otherwise, if type is "setter" and e.[[NeedsSet]] is true, then return.

  2. If IsPlatformObjectSameOrigin(platformObject) is false, then throw a "SecurityError" DOMException.

6.2.2. Shared internal slot: [[CrossOriginPropertyDescriptorMap]]

Window and Location objects both have a [[CrossOriginPropertyDescriptorMap]] internal slot, whose value is initially an empty map.

The [[CrossOriginPropertyDescriptorMap]] internal slot contains a map with entries whose keys are (currentOrigin, objectOrigin, propertyKey)-tuples and values are property descriptors, as a memoization of what is visible to scripts when currentOrigin inspects a Window or Location object from objectOrigin. It is filled lazily by CrossOriginGetOwnPropertyHelper, which consults it on future lookups.

User agents should allow a value held in the map to be garbage collected along with its corresponding key when nothing holds a reference to any part of the value. That is, as long as garbage collection is not observable.

For example, with
const href = Object.getOwnPropertyDescriptor(crossOriginLocation, "href").set

the value and its corresponding key in the map cannot be garbage collected as that would be observable.

User agents may have an optimization whereby they remove key-value pairs from the map when document.domain is set. This is not observable as document.domain cannot revisit an earlier value.

For example, setting document.domain to "example.com" on www.example.com means user agents can remove all key-value pairs from the map where part of the key is www.example.com, as that can never be part of the origin again and therefore the corresponding value could never be retrieved from the map.

6.2.3. Shared abstract operations

6.2.3.1. CrossOriginProperties ( O )
  1. Assert: O is a Location or Window object.

  2. If O is a Location object, then return

      « {
          [[Property]]: "href",
          [[NeedsGet]]: false,
          [[NeedsSet]]: true
        },
        {
          [[Property]]: "replace"
        } »

  3. Let crossOriginWindowProperties be

      « {
          [[Property]]: "window",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "self",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "location",
          [[NeedsGet]]: true,
          [[NeedsSet]]: true
        },
        {
          [[Property]]: "close"
        },
        {
          [[Property]]: "closed",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "focus"
        },
        {
          [[Property]]: "blur"
        },
        {
          [[Property]]: "frames",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "length",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "top",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "opener",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "parent",
          [[NeedsGet]]: true,
          [[NeedsSet]]: false
        },
        {
          [[Property]]: "postMessage"
        } »

  4. Repeat for each e that is an element of the child browsing context name property set:

    1. Add { [[Property]]: e } as the last element of crossOriginWindowProperties.

  5. Return crossOriginWindowProperties.

Indexed properties do not need to be safelisted as they are handled directly by the WindowProxy object.

6.2.3.2. IsPlatformObjectSameOrigin ( O )
  1. Return true if the current settings object’s origin is same origin-domain with O’s relevant settings object’s origin, and false otherwise.

6.2.3.3. CrossOriginGetOwnPropertyHelper ( O, P )

If this abstract operation returns undefined and there is no custom behavior, the caller needs to throw a "SecurityError" DOMException.

  1. If P is @@toStringTag, @@hasInstance, or @@isConcatSpreadable, then return PropertyDescriptor { [[Value]]: undefined, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

  2. Let crossOriginKey be a tuple consisting of the current settings object’s origin’s effective domain, O’s relevant settings object’s origin’s effective domain, and P.

  3. Repeat for each e that is an element of CrossOriginProperties(O):

    1. If SameValue(e.[[Property]], P) is true, then:

      1. If the value of the [[CrossOriginPropertyDescriptorMap]] internal slot of O contains an entry whose key is crossOriginKey, then return that entry’s value.

      2. Let originalDesc be OrdinaryGetOwnProperty(O, P).

      3. Let crossOriginDesc be CrossOriginPropertyDescriptor(e, originalDesc).

      4. Create an entry in the value of the [[CrossOriginPropertyDescriptorMap]] internal slot of O with key crossOriginKey and value crossOriginDesc.

      5. Return crossOriginDesc.

  4. Return undefined.

6.2.3.3.1. CrossOriginPropertyDescriptor ( crossOriginProperty, originalDesc )
  1. If crossOriginProperty.[[NeedsGet]] and crossOriginProperty.[[NeedsSet]] are absent, then:

    1. Let value be originalDesc.[[Value]].

    2. If IsCallable(value) is true, set value to CrossOriginFunctionWrapper(true, value).

    3. Return PropertyDescriptor{ [[Value]]: value, [[Enumerable]]: false, [[Writable]]: false, [[Configurable]]: true }.

  2. Otherwise:

    1. Let crossOriginGet be CrossOriginFunctionWrapper(crossOriginProperty.[[NeedsGet]], originalDesc.[[Get]]).

    2. Let crossOriginSet be CrossOriginFunctionWrapper(crossOriginProperty.[[NeedsSet]], originalDesc.[[Set]]).

    3. Return PropertyDescriptor{ [[Get]]: crossOriginGet, [[Set]]: crossOriginSet, [[Enumerable]]: false, [[Configurable]]: true }.

6.2.3.3.2. CrossOriginFunctionWrapper ( needsWrapping, functionToWrap )
  1. If needsWrapping is false, then return undefined.

  2. Return a new cross-origin wrapper function whose [[Wrapped]] internal slot is functionToWrap.

A cross-origin wrapper function is an anonymous built-in function that has a [[Wrapped]] internal slot.

When a cross-origin wrapper function F is called with a list of arguments argumentsList, the following steps are taken:

  1. Assert: F has a [[Wrapped]] internal slot that is a function.

  2. Let wrappedFunction be the [[Wrapped]] internal slot of F.

  3. Return Call(wrappedFunction, this, argumentsList).

Due to this being invoked from a different origin, a cross-origin wrapper function will have a different value for Function.prototype from the function being wrapped. This follows from how JavaScript creates anonymous built-in functions.

6.2.3.4. CrossOriginGet ( O, P, Receiver )
  1. Let desc be O.[[GetOwnProperty]](P).

  2. Assert: desc is not undefined.

  3. If IsDataDescriptor(desc) is true, then return desc.[[Value]].

  4. Assert: IsAccessorDescriptor(desc) is true.

  5. Let getter be desc.[[Get]].

  6. If getter is undefined, throw a "SecurityError" DOMException.

  7. Return Call(getter, Receiver).

6.2.3.5. CrossOriginSet ( O, P, V, Receiver )
  1. Let desc be O.[[GetOwnProperty]](P).

  2. Assert: desc is not undefined.

  3. If IsAccessorDescriptor(desc) is true, then:

    1. Let setter be desc.[[Set]].

    2. If setter is undefined, return false.

    3. Perform Call(setter, Receiver, «V»).

    4. Return true.

  4. Return false.

6.2.3.6. CrossOriginOwnPropertyKeys ( O )
  1. Let keys be a new empty List.

  2. Repeat for each e that is an element of CrossOriginProperties(O):

    1. Add e.[[Property]] as the last element of keys.

  3. Return keys.

6.3. The Window object

[PrimaryGlobal, LegacyUnenumerableNamedProperties]
/*sealed*/ interface Window : EventTarget {
// the current browsing context
[Unforgeable] readonly attribute WindowProxy window;
[Replaceable] readonly attribute WindowProxy self;
[Unforgeable] readonly attribute Document document;
attribute DOMString name;
[PutForwards=href, Unforgeable] readonly attribute Location location;
readonly attribute History history;
[Replaceable] readonly attribute BarProp locationbar;
[Replaceable] readonly attribute BarProp menubar;
[Replaceable] readonly attribute BarProp personalbar;
[Replaceable] readonly attribute BarProp scrollbars;
[Replaceable] readonly attribute BarProp statusbar;
[Replaceable] readonly attribute BarProp toolbar;
attribute DOMString status;
void close();
readonly attribute boolean closed;
void stop();
void focus();
void blur();

// other browsing contexts
[Replaceable] readonly attribute WindowProxy frames;
[Replaceable] readonly attribute unsigned long length;
[Unforgeable] readonly attribute WindowProxy top;
attribute any opener;
[Replaceable] readonly attribute WindowProxy parent;
readonly attribute Element? frameElement;
WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = "_blank", [TreatNullAs=EmptyString] optional DOMString features = "", optional boolean replace = false);
getter WindowProxy (unsigned long index);
getter object (DOMString name);
// Since this is the global object, the IDL named getter adds a NamedPropertiesObject exotic
// object on the prototype chain. Indeed, this does not make the global object an exotic object.
// Indexed access is taken care of by the {{WindowProxy}} exotic object.

// the user agent
readonly attribute Navigator navigator;

// user prompts
void alert();
void alert(DOMString message);
boolean confirm(optional DOMString message = "");
DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
void print();
any showModalDialog(DOMString url, optional any argument); // deprecated

unsigned long requestAnimationFrame(FrameRequestCallback callback);
void cancelAnimationFrame(unsigned long handle);
};
Window implements GlobalEventHandlers;
Window implements WindowEventHandlers;

callback FrameRequestCallback = void (DOMHighResTimeStamp time);
window . window
window . frames
window . self

These attributes all return window.

window . document

Returns the Document associated with window.

document . defaultView

Returns the Window object of the active document.

The window, frames, and self IDL attributes must all return the Window object’s browsing context’s WindowProxy object.

The document IDL attribute must return the Window object’s newest Document object.

The Document object associated with a Window object can change in exactly one case: when the navigate algorithm initializes a new Document object for the first page loaded in a browsing context. In that specific case, the Window object of the original about:blank page is reused and gets a new Document object.

The defaultView IDL attribute of the Document interface must return the Document's browsing context’s WindowProxy object, if there is one, or null otherwise.


For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named HTMLDocument whose value is the Document interface object.

6.3.1. APIs for creating and navigating browsing contexts by name

window = window . open( [ url [, target [, features [, replace ] ] ] ] )

Opens a window to show url (defaults to about:blank), and returns it. The target argument gives the name of the new window. If a window exists with that name already, it is reused. The replace attribute, if true, means that whatever page is currently open in that window will be removed from the window’s session history. The features argument can be used to influence the rendering of the new window.

window . name [ = value ]

Returns the name of the window.

Can be set, to change the name.

window . close()

Closes the window.

window . closed

Returns true if the window has been closed, false otherwise.

window . stop()

Cancels the document load.

The open() method on Window objects provides a mechanism for navigating an existing browsing context or opening and navigating an auxiliary browsing context.

When the method is invoked, the user agent must run the following steps:

  1. Let entry settings be the entry settings object when the method was invoked.
  2. Let url be the first argument.
  3. Let target be the second argument.
  4. Let features be the third argument.
  5. Let replace be the fourth argument.
  6. Let source browsing context be the responsible browsing context specified by entry settings.
  7. If target is the empty string, let it be the string "_blank" instead.
  8. If the user has indicated a preference for which browsing context to navigate, follow these substeps:

    1. Let target browsing context be the browsing context indicated by the user.
    2. If target browsing context is a new top-level browsing context, let the source browsing context be set as target browsing context’s one permitted sandboxed navigator.

    For example, suppose there is a user agent that supports control-clicking a link to open it in a new tab. If a user clicks in that user agent on an element whose onclick handler uses the window.open() API to open a page in an iframe, but, while doing so, holds the control key down, the user agent could override the selection of the target browsing context to instead target a new tab.

    Otherwise, apply the rules for choosing a browsing context given a browsing context name using target as the name and source browsing context as the context in which the algorithm is executed. If this results in there not being a chosen browsing context, then throw an InvalidAccessError exception and abort these steps. Otherwise, let target browsing context be the browsing context so obtained.

  9. If target browsing context was just created, either as part of the rules for choosing a browsing context given a browsing context name or due to the user indicating a preference for navigating a new top-level browsing context, then let new be true. Otherwise, let it be false.
  10. Interpret features as defined in the CSSOM View specification. [CSSOM-VIEW]
  11. If url is the empty string, run the appropriate steps from the following list:

    If new is false
    Jump to the step labeled end.
    If new is true
    Let resource be the URL "about:blank".

    Otherwise, parse url relative to entry settings, and let resource be the resulting URL record, if any. If the parse a URL algorithm failed, then run one of the following two steps instead:

    • Let resource be a resource representing an inline error page.
    • If new is false, jump to the step labeled end, otherwise, let resource be the URL "about:blank".
  12. If resource is "about:blank" and new is true, queue a task to fire a simple event named load at target browsing context’s Window object, with target override set to target browsing context’s Window object’s Document object.

    Otherwise, navigate target browsing context to resource, with the exceptions enabled flag set. If new is true, then replacement must be enabled also. The source browsing context is source browsing context.

  13. End: Return the WindowProxy object of target browsing context.

The name attribute of the Window object must, on getting, return the current name of the browsing context, if one is set, or the empty string otherwise; and, on setting, set the name of the browsing context to the new value.

The name gets reset when the browsing context is navigated to another domain.


The close() method on Window objects should, if all the following conditions are met, close the browsing context A:

A browsing context is script-closable if it is an auxiliary browsing context that was created by a script (as opposed to by an action of the user), or if it is a top-level browsing context whose session history contains only one Document.

The closed attribute on Window objects must return true if the Window object’s browsing context has been discarded, and false otherwise.

The stop() method on Window objects should, if there is an existing attempt to navigate the browsing context and that attempt is not currently running the unload a document algorithm, cancel that navigation; then, it must abort the active document of the browsing context of the Window object on which it was invoked.

6.3.2. Accessing other browsing contexts

window . length
Returns the number of child browsing contexts.
window[index]
Returns the indicated child browsing context.

The number of child browsing contexts of a Window object W is the number of child browsing contexts that are nested through elements that are in a Document that is the active document of the Window object’s associated Document object’s browsing context.

The length IDL attribute’s getter must return the number of child browsing contexts of this Window object.

Indexed access to child browsing contexts is defined through the [[GetOwnProperty]] internal method of the WindowProxy object.

6.3.3. Named access on the Window object

window[name]
Returns the indicated element or collection of elements.

As a general rule, relying on this will lead to brittle code. Which IDs end up mapping to this API can vary over time, as new features are added to the Web platform, for example. Instead of this, use document.getElementById() or document.querySelector().

The child browsing context name property set consists of the browsing context names of any child browsing context of the active document whose name is not the empty string, with duplicates omitted.

The Window interface supports named properties. The supported property names at any moment consist of the following, in tree order, ignoring later duplicates:

To determine the value of a named property name when the Window object is indexed for property retrieval, the user agent must return the value obtained using the following steps:

  1. Let objects be the list of named objects with the name name in the active document.

    There will be at least one such object, by definition.

  2. If objects contains a nested browsing context, then return the WindowProxy object of the nested browsing context corresponding to the first browsing context container in tree order whose browsing context is in objects, and abort these steps.

  3. Otherwise, if objects has only one element, return that element and abort these steps.

  4. Otherwise return an HTMLCollection rooted at the Document node, whose filter matches only named objects with the name name. (By definition, these will all be elements.)

Named objects with the name name, for the purposes of the above algorithm, are those that are either:

6.3.4. Garbage collection and browsing contexts

A browsing context has a strong reference to each of its Documents and its WindowProxy object, and the user agent itself has a strong reference to its top-level browsing contexts.

A Document has a strong reference to its Window object.

A Window object has a strong reference to its Document object through its document attribute. Thus, references from other scripts to either of those objects will keep both alive. Similarly, both Document and Window objects have implied strong references to the WindowProxy object.

Each script has a strong reference to its settings object, and each environment settings object has strong references to its global object, responsible browsing context, and responsible document (if any).

When a browsing context is to discard a Document, the user agent must run the following steps:

  1. Set the Document's salvageable state to false.
  2. Run any unloading document cleanup steps for the Document that are defined by this specification and other applicable specifications.
  3. Abort the Document.
  4. Remove any tasks associated with the Document in any task source, without running those tasks.
  5. Discard all the child browsing contexts of the Document.
  6. Lose the strong reference from the Document's browsing context to the Document.

Whenever a Document object is discarded, it is also removed from the list of the worker’s Documents of each worker whose list contains that Document.

When a browsing context is discarded, the strong reference from the user agent itself to the browsing context must be severed, and all the Document objects for all the entries in the browsing context’s session history must be discarded as well.

User agents may discard top-level browsing contexts at any time (typically, in response to user requests, e.g., when a user force-closes a window containing one or more top-level browsing contexts). Other browsing contexts must be discarded once their WindowProxy object is eligible for garbage collection.

6.3.5. Closing browsing contexts

When the user agent is required to close a browsing context, it must run the following steps:

  1. Let specified browsing context be the browsing context being closed.
  2. Prompt to unload the active document of the specified browsing context. If the user refused to allow the document to be unloaded, then abort these steps.
  3. Unload the active document of the specified browsing context with the recycle parameter set to false.
  4. Remove the specified browsing context from the user interface (e.g., close or hide its tab in a tabbed browser).
  5. Discard the specified browsing context.

User agents should offer users the ability to arbitrarily close any top-level browsing context.

6.3.6. Browser interface elements

To allow Web pages to integrate with Web browsers, certain Web browser interface elements are exposed in a limited way to scripts in Web pages.

Each interface element is represented by a BarProp object:

interface BarProp {
  readonly attribute boolean visible;
};
window . locationbar . visible
Returns true if the location bar is visible; otherwise, returns false.
window . menubar . visible
Returns true if the menu bar is visible; otherwise, returns false.
window . personalbar . visible
Returns true if the personal bar is visible; otherwise, returns false.
window . scrollbars . visible
Returns true if the scroll bars are visible; otherwise, returns false.
window . statusbar . visible
Returns true if the status bar is visible; otherwise, returns false.
window . toolbar . visible
Returns true if the toolbar is visible; otherwise, returns false.

The visible attribute, on getting, must return either true or a value determined by the user agent to most accurately represent the visibility state of the user interface element that the object represents, as described below.

The following BarProp objects exist for each Document object in a browsing context. Some of the user interface elements represented by these objects might have no equivalent in some user agents; for those user agents, except when otherwise specified, the object must act as if it was present and visible (i.e., its visible attribute must return true).

The location bar BarProp object
Represents the user interface element that contains a control that displays the URL of the active document, or some similar interface concept.
The menu bar BarProp object
Represents the user interface element that contains a list of commands in menu form, or some similar interface concept.
The personal bar BarProp object
Represents the user interface element that contains links to the user’s favorite pages, or some similar interface concept.
The scrollbar BarProp object
Represents the user interface element that contains a scrolling mechanism, or some similar interface concept.
The status bar BarProp object
Represents a user interface element found immediately below or after the document, as appropriate for the user’s media, which typically provides information about ongoing network activity or information about elements that the user’s pointing device is current indicating. If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e., its visible attribute may return false).
The toolbar BarProp object
Represents the user interface element found immediately above or before the document, as appropriate for the user’s media, which typically provides session history traversal controls (back and forward buttons, reload buttons, etc). If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e., its visible attribute may return false).

The locationbar attribute must return the location bar BarProp object.

The menubar attribute must return the menu bar BarProp object.

The personalbar attribute must return the personal bar BarProp object.

The scrollbars attribute must return the scrollbar BarProp object.

The statusbar attribute must return the status bar BarProp object.

The toolbar attribute must return the toolbar BarProp object.


For historical reasons, the status attribute on the Window object must, on getting, return the last string it was set to, and on setting, must set itself to the new value. When the Window object is created, the attribute must be set to the empty string. It does not do anything else.

6.3.7. The WindowProxy object

A WindowProxy is an exotic object that wraps a Window ordinary object, indirecting most operations through to the wrapped object. Each browsing context has an associated WindowProxy object. When the browsing context is navigated, the Window object wrapped by the browsing context’s associated WindowProxy object is changed.

There is no WindowProxy interface object.

Every WindowProxy object has a [[Window]] internal slot representing the wrapped Window object.

Although WindowProxy is named as a "proxy", it does not do polymorphic dispatch on its target’s internal methods as a real proxy would, due to a desire to reuse machinery between WindowProxy and Location objects. As long as the Window object remains an ordinary object this is unobservable and can be implemented either way.

In the following example, the variable x is set to the WindowProxy object returned by the window accessor on the global object. All of the expressions following the assignment return true, because the WindowProxy object passes most operations through to the underlying ordinary Window object.
var x = window;
x instanceof Window; // true
x === this; // true
6.3.7.1. The WindowProxy internal methods

The WindowProxy object internal methods are described in the subsections below.

6.3.7.1.1. [[GetPrototypeOf]] ( )
  1. Let W be the value of the [[Window]] internal slot of this.

  2. If IsPlatformObjectSameOrigin(W) is true, then return ! OrdinaryGetPrototypeOf(W).

  3. Return null.

6.3.7.1.2. [[SetPrototypeOf]] ( V )
  1. Return false.

6.3.7.1.3. [[IsExtensible]] ( )
  1. Return true.

6.3.7.1.4. [[PreventExtensions]] ( )
  1. Return false.

6.3.7.1.5. [[GetOwnProperty]] ( P )
  1. Let W be the value of the [[Window]] internal slot of this.

  2. If P is an array index property name, then:

    1. Let index be ToUint32(P).

    2. Let maxProperties be the number of child browsing contexts of W.

    3. Let value be undefined.

    4. If maxProperties is greater than 0 and index is less than maxProperties, then:

      1. Set value to the WindowProxy object of the indexth child browsing context of the Document that is nested through an element that is in W’s Document, sorted in the order that the elements nesting those browsing contexts were most recently inserted into the Document, the WindowProxy object of the most recently inserted browsing context container’s nested browsing context being last.

    5. Return PropertyDescriptor{ [[Value]]: value, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

  3. If IsPlatformObjectSameOrigin(W) is true, then return OrdinaryGetOwnProperty(W, P).

    This violates JavaScript’s internal method invariants.

  4. Let property be CrossOriginGetOwnPropertyHelper(W, P).

  5. If property is not undefined, return property.

  6. If property is undefined and P is in the child browsing context name property set, then:

    1. Let value be the WindowProxy object of the named object with the name P.

    2. Return PropertyDescriptor{ [[Value]]: value, [[Enumerable]]: false, [[Writable]]: false, [[Configurable]]: true }.

  7. Throw a "SecurityError" DOMException.

6.3.7.1.6. [[DefineOwnProperty]] ( P, Desc )
  1. If P is an array index property name, return false.

  2. Let W be the value of the [[Window]] internal slot of this.

  3. If IsPlatformObjectSameOrigin(W) is true, then return OrdinaryDefineOwnProperty(W, P, Desc).

    See above about how this violates JavaScript’s internal method invariants.

  4. Return false.

6.3.7.1.7. [[Get]] ( P, Receiver )
  1. Let W be the value of the [[Window]] internal slot of this.

  2. If IsPlatformObjectSameOrigin(W) is true, then return OrdinaryGet(this, P, Receiver).

  3. Return ? CrossOriginGet(this, P, Receiver).

6.3.7.1.8. [[Set]] ( P, V, Receiver )
  1. Let W be the value of the [[Window]] internal slot of this.

  2. If IsPlatformObjectSameOrigin(W) is true, then return OrdinarySet(W, this, Receiver).

  3. Return CrossOriginSet(this, P, V, Receiver).

6.3.7.1.9. [[Delete]] ( P )
  1. If P is an array index property name, return false.

  2. Let W be the value of the [[Window]] internal slot of this.

  3. If IsPlatformObjectSameOrigin(W) is true, then return OrdinaryDelete(W, P).

  4. Return false.

6.3.7.1.10. [[OwnPropertyKeys]] ( )
  1. Let W be the value of the [[Window]] internal slot of this.

  2. Let keys be a new empty List.

  3. Let maxProperties be the number of child browsing contexts of W.

  4. Let index be 0.

  5. Repeat while index < maxProperties,

    1. Add ! ToString(index) as the last element of keys.

    2. Increment index by 1.

  6. If IsPlatformObjectSameOrigin(W) is true, then return the concatenation of keys and ! OrdinaryOwnPropertyKeys(W).

  7. Return the concatenation of keys and ! CrossOriginOwnPropertyKeys(W).

6.4. Origin

Origins are the fundamental currency of the Web’s security model. Two actors in the Web platform that share an origin are assumed to trust each other and to have the same authority. Actors with differing origins are considered potentially hostile versus each other, and are isolated from each other to varying degrees.

For example, if Example Bank’s Web site, hosted at bank.example.com, tries to examine the DOM of Example Charity’s Web site, hosted at charity.example.org, a "SecurityError" DOMException will be raised.


An origin is one of the following:

An opaque origin

An internal value, with no serialisation, for which the only meaningful operation is testing for equality.

A tuple origin

A tuple consists of:

  • A scheme (a scheme).

  • A host (a host).

  • A port (a port).

  • A domain (null or a domain). Null unless stated otherwise.

Origins can be shared, e.g., among multiple Document objects. Furthermore, origins are generally immutable. Only the domain of a tuple origin can be changed, and only through the document.domain API.

The effective domain of an origin origin is computed as follows:

  1. If origin is an opaque origin, then return origin.

  2. If origin’s domain is non-null, then return origin’s domain.

  3. Return origin’s host.

Various specification objects are defined to have an origin. These origins are determined as follows:

For Document objects

If the Document's active sandboxing flag set has its sandboxed origin browsing context flag set
If the Document was generated from a data: URL
A unique opaque origin is assigned when the Document is created.
If the Document's URL’s scheme is a network scheme
A copy of the Document's URL’s origin assigned when the Document is created.

The document.open() method can change the Document's URL to "about:blank". Therefore the origin is assigned when the Document is created.

If the Document is the initial "about:blank" document
The one it was assigned when its browsing context was created.
If the Document is a non-initial "about:blank" document
The origin of the incumbent settings object when the navigate algorithm was invoked, or, if no script was involved, of the node document of the element that initiated the navigation to that URL.
If the Document was created as part of the processing for javascript: URLs
The origin of the active document of the browsing context being navigated when the navigate algorithm was invoked.
If the Document is an iframe srcdoc document
The origin of the Document's browsing context’s browsing context container’s node document.
If the Document was obtained in some other manner (e.g., a Document created using the createDocument() API, etc)
The default behavior as defined in the DOM standard applies. [DOM].

The origin is a unique opaque origin assigned when the Document is created.

For images of img elements

If the image data is CORS-cross-origin
A unique opaque origin assigned when the image is created.
If the image data is CORS-same-origin
The img element’s node document’s origin.

For audio and video elements

If the media data is CORS-cross-origin
A unique opaque origin assigned when the media data is fetched.
If the media data is CORS-same-origin
The media element’s node document’s origin.

For fonts

For a downloadable Web font it is a copy of the origin of the URL record used to obtain the font (after any redirects). [CSS-FONTS-3] [CSS-FONT-LOADING-3]

For a locally installed system font it is the origin of the Document in which that font is being used.

Other specifications can override the above definitions by themselves specifying the origin of a particular Document object, image, media element, or font.


The Unicode serialization of an origin is the string obtained by applying the following algorithm to the given origin origin:

  1. If origin is an opaque origin, then return "null".

  2. Let host be origin’s host.

  3. Let unicodeHost be host if host is not a domain, and the result of applying domain to Unicode to host otherwise.

  4. Let unicodeOrigin be a new tuple origin consisting origin’s scheme, unicodeHost, and origin’s port.

  5. Return the ASCII serialization of an origin, given unicodeOrigin.

    The name ASCII serialization of an origin is misleading, as it merely serialises an origin, which are all ASCII by default due to the URL parser.

The Unicode serialization of ("https", "xn--maraa-rta.example", null, null) is "https://maraña.example".

The ASCII serialization of an origin is the string obtained by applying the following algorithm to the given origin origin:

  1. If origin is an opaque origin, then return "null".

  2. Otherwise, let result be origin’s scheme.

  3. Append "://" to result.

  4. Append origin’s host, serialized, to result.

  5. If origin’s port is non-null, append a U+003A COLON character (:), and origin’s port, serialized, to result.

  6. Return result.

Two origins A and B are said to be same origin if the following algorithm returns true:

  1. If A and B are the same opaque origin, then return true.

  2. If A and B are both tuple origins, and their schemes, hosts, and ports are identical, then return true.

  3. Return false.

Two origins A and B are said to be same origin-domain if the following algorithm returns true:

  1. If A and B are the same opaque origin, then return true.

  2. If A and B are both tuple origins, run these substeps:

    1. If A and B’s schemes are identical, and their domains are identical and non-null, then return true.

    2. Otherwise, if A and B are same origin and their domains are identical and null, then return true.

  3. Return false.

The following table shows how A and B are related:
A B same origin same origin-domain
("https", "example.org", null, null) ("https", "example.org", null, null)
("https", "example.org", 314, "example.org") ("https", "example.org", 420, "example.org")
("https", "example.org", null, null) ("https", "example.org", null, "example.org")
("https", "example.org", null, "example.org") ("http", "example.org", null, "example.org")

6.4.1. Relaxing the same-origin restriction

document . domain [ = domain ]
Returns the current domain used for security checks.

Can be set to a value that removes subdomains, to change the origin’s domain to allow pages on other subdomains of the same domain (if they do the same thing) to access each other. (Can’t be set in sandboxed iframes.)

The domain attribute’s getter must run these steps:

  1. If this Document object does not have a browsing context, then return the empty string.

  2. Let effectiveDomain be this Document's origin’s effective domain.

  3. If effectiveDomain is an opaque origin, then return the empty string.

  4. Return effectiveDomain, serialised.

The domain attribute on setting must run these steps:

  1. If this Document object has no browsing context, throw a "SecurityError" DOMException.

  2. If this Document object’s active sandboxing flag set has its sandboxed document.domain browsing context flag set, then throw a "SecurityError" DOMException.

  3. If the given value is the empty string, then throw a "SecurityError" DOMException.

  4. Let host be the result of parsing the given value.

  5. If host is failure, then throw a "SecurityError" DOMException.

  6. Let effectiveDomain be this Document object’s origin’s effective domain.

  7. If host is not equal to effectiveDomain, then run these substeps:

    1. If host or effectiveDomain is not domain, then throw a "SecurityError" DOMException.

      This is meant to exclude hosts that are an IPv4 address or an IPv6 address.

    2. If host, prefixed by a U+002E FULL STOP (.), does not exactly match the end of effectiveDomain, then throw a "SecurityError" DOMException.

    3. If host matches a suffix in the Public Suffix List, or, if host, prefixed by a U+002E FULL STOP (.), matches the end of a suffix in the Public Suffix List, then throw a "SecurityError" DOMException. [PSL]

      Suffixes must be compared after applying the host parser algorithm. [URL]

  8. Set origin’s domain to host.

The document.domain attribute is used to enable pages on different hosts of a domain to access each others' DOMs.

Do not use the document.domain attribute when using shared hosting. If an untrusted third party is able to host an HTTP server at the same IP address but on a different port, then the same-origin protection that normally protects two different sites on the same host will fail, as the ports are ignored when comparing origins after the document.domain attribute has been used.

6.5. Sandboxing

A sandboxing flag set is a set of zero or more of the following flags, which are used to restrict the abilities that potentially untrusted resources have:

The sandboxed navigation browsing context flag

This flag prevents content from navigating browsing contexts other than the sandboxed browsing context itself (or browsing contexts further nested inside it), auxiliary browsing contexts (which are protected by the sandboxed auxiliary navigation browsing context flag defined next), and the top-level browsing context (which is protected by the sandboxed top-level navigation browsing context flag defined below).

If the sandboxed auxiliary navigation browsing context flag is not set, then in certain cases the restrictions nonetheless allow popups (new top-level browsing contexts) to be opened. These browsing contexts always have one permitted sandboxed navigator, set when the browsing context is created, which allows the browsing context that created them to actually navigate them. (Otherwise, the sandboxed navigation browsing context flag would prevent them from being navigated even if they were opened.)

The sandboxed auxiliary navigation browsing context flag

This flag prevents content from creating new auxiliary browsing contexts, e.g., using the target attribute, the window.open() method, or the showModalDialog() method.

The sandboxed top-level navigation browsing context flag

This flag prevents content from navigating their top-level browsing context and prevents content from closing their top-level browsing context.

When the sandboxed top-level navigation browsing context flag is not set, content can navigate its top-level browsing context, but other browsing contexts are still protected by the sandboxed navigation browsing context flag and possibly the sandboxed auxiliary navigation browsing context flag.

The sandboxed plugins browsing context flag

This flag prevents content from instantiating plugins, whether using the embed element, the object element, the applet element, or through navigation of a nested browsing context, unless those plugins can be secured.

The sandboxed origin browsing context flag

This flag forces content into a unique origin, thus preventing it from accessing other content from the same origin.

This flag also prevents script from reading from or writing to the document.cookie IDL attribute, and blocks access to localStorage. [WEBSTORAGE]

The sandboxed forms browsing context flag

This flag blocks form submission.

The sandboxed pointer lock browsing context flag

This flag disables the Pointer Lock API. [POINTERLOCK]

The sandboxed scripts browsing context flag

This flag blocks script execution.

The sandboxed automatic features browsing context flag

This flag blocks features that trigger automatically, such as automatically playing a video or automatically focusing a form control.

The sandboxed storage area URLs flag

This flag prevents URL schemes that use storage areas from being able to access the origin’s data.

The sandboxed fullscreen browsing context flag

This flag prevents content from using the requestFullscreen() method.

The sandboxed document.domain browsing context flag
This flag prevents content from using the document.domain setter.
The sandbox propagates to auxiliary browsing contexts flag

This flag prevents content from escaping the sandbox by ensuring that any auxiliary browsing context it creates inherits the content’s active sandboxing flag set.

The sandboxed modals flag

This flag prevents content from using any of the following features to produce modal dialogs:

  • window.alert()
  • window.confirm()
  • window.print()
  • window.prompt()
  • window.showModalDialog()
  • the beforeunload event

When the user agent is to parse a sandboxing directive, given a string input, a sandboxing flag set output, and optionally an allow fullscreen flag, it must run the following steps:

  1. Split input on spaces, to obtain tokens.
  2. Let output be empty.
  3. Add the following flags to output:


Every top-level browsing context has a popup sandboxing flag set, which is a sandboxing flag set. When a browsing context is created, its popup sandboxing flag set must be empty. It is populated by the rules for choosing a browsing context given a browsing context name.

Every nested browsing context has an iframe sandboxing flag set, which is a sandboxing flag set. Which flags in a nested browsing context’s iframe sandboxing flag set are set at any particular time is determined by the iframe element’s sandbox attribute.

Every Document has an active sandboxing flag set, which is a sandboxing flag set. When the Document is created, its active sandboxing flag set must be empty. It is populated by the navigation algorithm.

Every resource that is obtained by the navigation algorithm has a forced sandboxing flag set, which is a sandboxing flag set. A resource by default has no flags set in its forced sandboxing flag set, but other specifications can define that certain flags are set.

In particular, the forced sandboxing flag set is used by the Content Security Policy specification. [CSP3]


When a user agent is to implement the sandboxing for a Document, it must populate Document's active sandboxing flag set with the union of the flags that are present in the following sandboxing flag sets at the time the Document object is created:

6.6. Session history and navigation

6.6.1. The session history of browsing contexts

The sequence of Documents in a browsing context is its session history. Each browsing context, including nested browsing contexts, has a distinct session history. A browsing context’s session history consists of a flat list of session history entries. Each session history entry consists, at a minimum, of a URL, and each entry may in addition have a state object, a title, a Document object, form data, a scroll restoration mode, a scroll position, and other information associated with it.

Each entry, when first created, has a Document. However, when a Document is not active, it’s possible for it to be discarded to free resources. The URL and other data in a session history entry is then used to bring a new Document into being to take the place of the original, should the user agent find itself having to reactivate that Document.

Titles associated with session history entries need not have any relation with the current title of the Document. The title of a session history entry is intended to explain the state of the document at that point, so that the user can navigate the document’s history.

URLs without associated state objects are added to the session history as the user (or script) navigates from page to page.


Each Document object in a browsing context’s session history is associated with a unique History object which must all model the same underlying session history.

The history attribute of the Window interface must return the object implementing the History interface for that Window object’s newest Document.

A state object is an object representing a user interface state.

Pages can add state objects to the session history. These are then returned to the script when the user (or script) goes back in the history, thus enabling authors to use the "navigation" metaphor even in one-page applications.

State objects are intended to be used for two main purposes: first, storing a preparsed description of the state in the URL so that in the simple case an author doesn’t have to do the parsing (though one would still need the parsing for handling URLs passed around by users, so it’s only a minor optimization), and second, so that the author can store state that one wouldn’t store in the URL because it only applies to the current Document instance and it would have to be reconstructed if a new Document were opened.

An example of the latter would be something like keeping track of the precise coordinate from which a pop-up div was made to animate, so that if the user goes back, it can be made to animate to the same location. Or alternatively, it could be used to keep a pointer into a cache of data that would be fetched from the server based on the information in the URL, so that when going back and forward, the information doesn’t have to be fetched again.


At any point, one of the entries in the session history is the current entry. This is the entry representing the active document of the browsing context. Which entry is the current entry is changed by the algorithms defined in this specification, e.g., during session history traversal.

The current entry is usually an entry for the URL of the Document. However, it can also be one of the entries for state objects added to the history by that document.

An entry with persisted user state is one that also has user-agent defined state. This specification does not specify what kind of state can be stored.

For example, some user agents might want to persist the scroll position, or the values of form controls.

User agents that persist the value of form controls are encouraged to also persist their directionality (the value of the element’s dir attribute). This prevents values from being displayed incorrectly after a history traversal when the user had originally entered the values with an explicit, non-default directionality.

An entry’s scroll restoration mode indicates whether the user agent should restore the persisted scroll position (if any) when traversing to it. The scroll restoration mode may be one of the following:

"auto"
The user agent is responsible for restoring the scroll position upon navigation.
"manual"
The page is responsible for restoring the scroll position and the user agent does not attempt to do so automatically

If unspecified, the scroll restoration mode of a new entry must be set to "auto".

Entries that consist of state objects share the same Document as the entry for the page that was active when they were added.

Contiguous entries that differ just by fragment also share the same Document.

All entries that share the same Document (and that are therefore merely different states of one particular document) are contiguous by definition.

Each Document in a browsing context can also have a latest entry. This is the entry for that Document to which the browsing context’s session history was most recently traversed. When a Document is created, it initially has no latest entry.

User agents may discard the Document objects of entries other than the current entry that are not referenced from any script, reloading the pages afresh when the user or script navigates back to such pages. This specification does not specify when user agents should discard Document objects and when they should cache them.

Entries that have had their Document objects discarded must, for the purposes of the algorithms given below, act as if they had not. When the user or script navigates back or forwards to a page which has no in-memory DOM objects, any other entries that shared the same Document object with it must share the new object as well.

6.6.2. The History interface

enum ScrollRestoration { "auto", "manual" };
interface History {
  readonly attribute unsigned long length;
  attribute ScrollRestoration scrollRestoration;
  readonly attribute any state;
  void go(optional long delta = 0);
  void back();
  void forward();
  void pushState(any data, DOMString title, optional DOMString? url = null);
  void replaceState(any data, DOMString title, optional DOMString? url = null);
};
window . history . length

Returns the number of entries in the joint session history.

window . history . scrollRestoration [ = value ]

Returns the scroll restoration mode of the current entry in the session history.

Can be set, to change the scroll restoration mode of the current entry in the session history.

window . history . state

Returns the current state object.

window . history . go( [ delta ] )

Goes back or forward the specified number of steps in the joint session history.

A zero delta will reload the current page.

If the delta is out of range, does nothing.

window . history . back()

Goes back one step in the joint session history.

If there is no previous page, does nothing.

window . history . forward()

Goes forward one step in the joint session history.

If there is no next page, does nothing.

window . history . pushState(data, title [, url ] )

Pushes the given data onto the session history, with the given title, and, if provided and not null, the given URL.

window . history . replaceState(data, title [, url ] )

Updates the current entry in the session history to have the given data, title, and, if provided and not null, URL.

The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, with all the entries that are current entries in their respective session histories removed except for the current entry of the joint session history.

The current entry of the joint session history is the entry that most recently became a current entry in its session history.

Entries in the joint session history are ordered chronologically by the time they were added to their respective session histories. Each entry has an index; the earliest entry has index 0, and the subsequent entries are numbered with consecutively increasing integers (1, 2, 3, etc).

Since each Document in a browsing context might have a different event loop, the actual state of the joint session history can be somewhat nebulous. For example, two sibling iframe elements could both traverse from one unique origin to another at the same time, so their precise order might not be well-defined; similarly, since they might only find out about each other later, they might disagree about the length of the joint session history.

The length attribute of the History interface, on getting, must return the number of entries in the top-level browsing context’s joint session history. If this History object is associated with a Document that is not fully active, getting must instead throw a "SecurityError" DOMException.

The actual entries are not accessible from script.

The scrollRestoration attribute of the History interface, on getting, must return the scroll restoration mode of the current entry in the session history. On setting, the scroll restoration mode of the current entry in the session history must be set to the new value. If this History object is associated with a Document that is not fully active, both getting and setting must instead throw a "SecurityError" DOMException.

The state attribute of the History interface, on getting, must return the last value it was set to by the user agent. If this History object is associated with a Document that is not fully active, getting must instead throw a SecurityError DOMException. Initially, its value must be null.

When the go(delta) method is invoked, if delta is zero, the user agent must act as if the location.reload() method was called instead. Otherwise, the user agent must traverse the history by a delta whose value is delta If this History object is associated with a Document that is not fully active, invoking must instead throw a "SecurityError" DOMException.

When the back() method is invoked, the user agent must traverse the history by a delta −1. If this History object is associated with a Document that is not fully active, invoking must instead throw a "SecurityError" DOMException.

When the forward()method is invoked, the user agent must traverse the history by a delta +1. If this History object is associated with a Document that is not fully active, invoking must instead throw a "SecurityError" DOMException.


Each top-level browsing context has a session history traversal queue, initially empty, to which tasks can be added.

Each top-level browsing context, when created, must begin running the following algorithm, known as the session history event loop for that top-level browsing context, in parallel:

  1. Wait until this top-level browsing context’s session history traversal queue is not empty.
  2. Pull the first task from this top-level browsing context’s session history traversal queue, and execute it.
  3. Return to the first step of this algorithm.

The session history event loop helps coordinate cross-browsing-context transitions of the joint session history: since each browsing context might, at any particular time, have a different event loop (this can happen if the user agent has more than one event loop per unit of related browsing contexts), transitions would otherwise have to involve cross-event-loop synchronization.


To traverse the history by a delta delta, the user agent must append a task to this top-level browsing context’s session history traversal queue, the task consisting of running the following steps:

  1. If the index of the current entry of the joint session history plus delta is less than zero or greater than or equal to the number of items in the joint session history, then abort these steps.
  2. Let specified entry be the entry in the joint session history whose index is the sum of delta and the index of the current entry of the joint session history.
  3. Let specified browsing context be the browsing context of the specified entry.
  4. If the specified browsing context’s active document’s unload a document algorithm is currently running, abort these steps.
  5. Queue a task that consists of running the following substeps. The relevant event loop is that of the specified browsing context’s active document. The task source for the queued task is the history traversal task source.

    1. If there is an ongoing attempt to navigate specified browsing context that has not yet matured (i.e., it has not passed the point of making its Document the active document), then cancel that attempt to navigate the browsing context.
    2. If the specified browsing context’s active document is not the same Document as the Document of the specified entry, then run these substeps:

      1. Prompt to unload the active document of the specified browsing context. If the user refused to allow the document to be unloaded, then abort these steps.
      2. Unload the active document of the specified browsing context with the recycle parameter set to false.
    3. Traverse the history of the specified browsing context to the specified entry.

When the user navigates through a browsing context, e.g., using a browser’s back and forward buttons, the user agent must traverse the history by a delta equivalent to the action specified by the user.


The pushState() method adds a state object entry to the history.

The replaceState() method updates the state object, title, and optionally the URL of the current entry in the history.

When either of these methods is invoked, the user agent must run the following steps:

  1. If this History object is associated with a Document that is not fully active, throw a "SecurityError" DOMException.

  2. Optionally, abort these steps. (For example, the user agent might disallow calls to these methods that are invoked on a timer, or from event listeners that are not triggered in response to a clear user action, or that are invoked in rapid succession.)

  3. Let targetRealm be this History object’s relevant settings object’s Realm.

  4. Let cloned data be a StructuredClone(data, targetRealm). Rethrow any exceptions.

  5. If the third argument is not null, run these substeps:

    1. Parse the value of the third argument, relative to the entry settings object.

    2. If that fails, throw a "SecurityError" DOMException and abort these steps.

    3. Let new URL be the resulting URL record.

    4. Compare new URL to the document’s URL. If any component of these two URL records differ other than the path, query, and fragment components, then throw a "SecurityError" DOMException and abort these steps.

    5. If the origin of new URL is not the same as the origin of the responsible document specified by the entry settings object, and either the path or query components of the two URL records compared in the previous step differ, throw a "SecurityError" DOMException and abort these steps. (This prevents sandboxed content from spoofing other pages on the same origin.)

  6. If the third argument is null, then let new URL be the URL of the current entry.

  7. If the method invoked was the pushState() method:

    1. Remove all the entries in the browsing context’s session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

      This doesn’t necessarily have to affect the user agent’s user interface.

    2. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context’s document family.

    3. If appropriate, update the current entry to reflect any state that the user agent wishes to persist. The entry is then said to be an entry with persisted user state.

    4. Add a state object entry to the session history, after the current entry, with cloned data as the state object, the given title as the title, new URL as the URL of the entry, and the scroll restoration mode of the current entry in the session history as the scroll restoration mode.

    5. Update the current entry to be this newly added entry.

    Otherwise, if the method invoked was the replaceState() method:

    1. Update the current entry in the session history so that cloned data is the entry’s new state object, the given title is the new title, and new URL is the entry’s new URL.

  8. If the current entry in the session history represents a non-GET request (e.g., it was the result of a POST submission) then update it to instead represent a GET request.

  9. Set the document’s URL to new URL.

    Since this is neither a navigation of the browsing context nor a history traversal, it does not cause a hashchange event to be fired.

  10. Let targetRealm be this History object’s relevant settings object’s Realm.

  11. Set history.state to StructuredClone(cloned data, targetRealm).

  12. Let the latest entry of the Document of the current entry be the current entry.

The title is purely advisory. User agents might use the title in the user interface.

User agents may limit the number of state objects added to the session history per page. If a page hits the user agent-defined limit, user agents must remove the entry immediately after the first entry for that Document object in the session history after having added the new entry. (Thus the state history acts as a FIFO buffer for eviction, but as a LIFO buffer for navigation.)

Consider a game where the user can navigate along a line, such that the user is always at some coordinate, and such that the user can bookmark the page corresponding to a particular coordinate, to return to it later.

A static page implementing the x=5 position in such a game could look like the following:

<!DOCTYPE HTML>
<!-- this is https://example.com/line?x=5 -->
<title>Line Game - 5</title>
<p>You are at coordinate 5 on the line.</p>
<p>
<a href="?x=6">Advance to 6</a> or
<a href="?x=4">retreat to 4</a>?
</p>

The problem with such a system is that each time the user clicks, the whole page has to be reloaded. Here instead is another way of doing it, using script:

<!DOCTYPE HTML>
<!-- this starts off as https://example.com/line?x=5 -->
<title>Line Game - 5</title>
<p>You are at coordinate <span>5</span> on the line.</p>
<p>
<a href="?x=6" onclick="go(1); return false;">Advance to 6</a> or
<a href="?x=4" onclick="go(-1); return false;">retreat to 4</a>?
</p>
<script>
var currentPage = 5; // prefilled by server
function go(d) {
  setupPage(currentPage + d);
  history.pushState(currentPage, document.title, '?x=' + currentPage);
}
onpopstate = function(event) {
  setupPage(event.state);
}
function setupPage(page) {
  currentPage = page;
  document.title = 'Line Game - ' + currentPage;
  document.getElementById('coord').textContent = currentPage;
  document.links[0].href = '?x=' + (currentPage+1);
  document.links[0].textContent = 'Advance to ' + (currentPage+1);
  document.links[1].href = '?x=' + (currentPage-1);
  document.links[1].textContent = 'retreat to ' + (currentPage-1);
}
</script>

In systems without script, this still works like the previous example. However, users that do have script support can now navigate much faster, since there is no network access for the same experience. Furthermore, contrary to the experience the user would have with just a naïve script-based approach, bookmarking and navigating the session history still work.

In the example above, the data argument to the pushState() method is the same information as would be sent to the server, but in a more convenient form, so that the script doesn’t have to parse the URL each time the user navigates.

Applications might not use the same title for a session history entry as the value of the document’s title element at that time. For example, here is a simple page that shows a block in the title element. Clearly, when navigating backwards to a previous state the user does not go back in time, and therefore it would be inappropriate to put the time in the session history title.
<!DOCTYPE HTML>
<TITLE>Line</TITLE>
<SCRIPT>
setInterval(function () { document.title = 'Line - ' + new Date(); }, 1000);
var i = 1;
function inc() {
  set(i+1);
  history.pushState(i, 'Line - ' + i);
}
function set(newI) {
  i = newI;
  document.forms.F.I.value = newI;
}
</SCRIPT>
<BODY ONPOPSTATE="set(event.state)">
<FORM NAME=F>
State: <OUTPUT NAME=I>1</OUTPUT> <INPUT VALUE="Increment" TYPE=BUTTON ONCLICK="inc()">
</FORM>
Most applications want to use the same scroll restoration mode value for all of their history entries. To achieve this they should set the scrollRestoration attribute as soon as possible (e.g., in the first script element in the document’s head element) to ensure that any entry added to the history session gets the desired scroll restoration mode.
<head>
  <script>
    if ('scrollRestoration' in history)
      history.scrollRestoration = 'manual';
  </script>
</head>

6.6.3. Implementation notes for session history

This section is non-normative.

The History interface is not meant to place restrictions on how implementations represent the session history to the user.

For example, session history could be implemented in a tree-like manner, with each page having multiple "forward" pages. This specification doesn’t define how the linear list of pages in the history object are derived from the actual session history as seen from the user’s perspective.

Similarly, a page containing two iframes has a history object distinct from the iframes' history objects, despite the fact that typical Web browsers present the user with just one "Back" button, with a session history that interleaves the navigation of the two inner frames and the outer page.

Security: It is suggested that to avoid letting a page "hijack" the history navigation facilities of a UA by abusing pushState(), the UA provide the user with a way to jump back to the previous page (rather than just going back to the previous state). For example, the back button could have a drop down showing just the pages in the session history, and not showing any of the states. Similarly, an aural browser could have two "back" commands, one that goes back to the previous state, and one that jumps straight back to the previous page.

For both pushState() and replaceState(), user agents are encouraged to prevent abuse of these APIs via too-frequent calls or over-large state objects. As detailed above, the algorithm explicitly allows user agents to ignore any such calls when appropriate.

6.6.4. The Location interface

Each Window object is associated with a unique instance of a Location object, allocated when the Window object is created.

To create a Location object, run these steps:

  1. Let location be a new Location platform object.

  2. Perform ! location.[[DefineOwnProperty]]("toString", { [[Value]]: %ObjProto_toString%, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).

  3. Perform ! location.[[DefineOwnProperty]]("toJSON", { [[Value]]: undefined, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).

  4. Perform ! location.[[DefineOwnProperty]]("valueOf", { [[Value]]: %ObjProto_valueOf%, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).

  5. Perform ! location.[[DefineOwnProperty]](@@toPrimitive, { [[Value]]: undefined, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).

  6. Set the value of the [[DefaultProperties]] internal slot of location to location.[[OwnPropertyKeys]]().

  7. Return location.

document . location [ = value ]
window . location [ = value ]
Returns a Location object with the current page’s location.

Can be set, to navigate to another page.

The location attribute of the Document interface must return the Location object for that Document object’s global object, if it has a browsing context, and null otherwise.

The location attribute of the Window interface must return the Location object for that Window object.

Location objects provide a representation of the URL of the active document of their Document's browsing context, and allow the current entry of the browsing context’s session history to be changed, by adding or replacing entries in the history object.

interface Location {
  [Unforgeable] stringifier attribute USVString href;
  [Unforgeable] readonly attribute USVString origin;
  [Unforgeable] attribute USVString protocol;
  [Unforgeable] attribute USVString host;
  [Unforgeable] attribute USVString hostname;
  [Unforgeable] attribute USVString port;
  [Unforgeable] attribute USVString pathname;
  [Unforgeable] attribute USVString search;
  [Unforgeable] attribute USVString hash;

  [Unforgeable] void assign(USVString url);
  [Unforgeable] void replace(USVString url);
  [Unforgeable] void reload();

  [Unforgeable, SameObject] readonly attribute USVString[] ancestorOrigins;
};
location . toString()
location . href
Returns the Location object’s URL.

Can be set, to navigate to the given URL.

location . origin
Returns the Location object’s URL’s origin.
location . protocol
Returns the Location object’s URL’s scheme.

Can be set, to navigate to the same URL with a changed scheme.

location . host
Returns the Location object’s URL’s host and port (if different from the default port for the scheme).

Can be set, to navigate to the same URL with a changed host and port.

location . hostname
Returns the Location object’s URL’s host.

Can be set, to navigate to the same URL with a changed host.

location . port
Returns the Location object’s URL’s port.

Can be set, to navigate to the same URL with a changed port.

location . pathname
Returns the Location object’s URL’s path.

Can be set, to navigate to the same URL with a changed path.

location . search
Returns the Location object’s URL’s query (includes leading "?" if non-empty).

Can be set, to navigate to the same URL with a changed query (ignores leading "?").

location . hash
Returns the Location object’s URL’s fragment (includes leading "#" if non-empty).

Can be set, to navigate to the same URL with a changed fragment (ignores leading "#").

location . assign(url)
Navigates to the given URL.
location . replace(url)
Removes the current page from the session history and navigates to the given URL.
location . reload()
Reloads the current page.
location . ancestorOrigins
Returns an array whose values are the origins of the ancestor browsing contexts, from the parent browsing context to the top-level browsing context.

A Location object has an associated relevant Document, which is this Location object’s associated Document object’s browsing context’s active document.

A Location object has an associated url, which is this Location object’s relevant Document’s URL.

A Location object has an associated ancestor origins array. When a Location object is created, its ancestor origins array must be set to a array created from the list of strings that the following steps would produce:

  1. Let output be an empty ordered list of strings.

  2. Let current be the browsing context of the Document with which the Location object is associated.

  3. Loop: If current has no parent browsing context, jump to the step labeled End.

  4. Let current be current’s parent browsing context.

  5. Append the Unicode serialization of current’s active document’s origin to output as a new value.

  6. Return to the step labeled Loop.

  7. End: Return output.

A Location object has an associated Location-object-setter navigate algorithm, which given a url, runs these steps:

  1. If any of the following conditions are met, let replacement flag be unset; otherwise, let it be set:

  2. Location-object navigate, given url and replacement flag.

To Location-object navigate, given a url and replacement flag, run these steps:

  1. The source browsing context is the responsible browsing context specified by the incumbent settings object.

  2. Navigate the browsing context to url, with the exceptions enabled flag set. Rethrow any exceptions.

    If the replacement flag is set or the browsing context’s session history contains only one Document, and that was the about:blank Document created when the browsing context was created, then the navigation must be done with replacement enabled.

The href attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Return this Location object’s URL, serialized.

The href attribute’s setter must run these steps:

  1. Parse the given value relative to the entry settings object. If that failed, throw a TypeError exception.

  2. Location-object-setter navigate to the resulting URL record.

The href attribute setter intentionally has no security check.

The origin attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Return the Unicode serialization of this Location object’s URL’s origin.

It returns the Unicode rather than the ASCII serialization for compatibility with MessageEvent.

The protocol attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Return this Location object’s URL’s scheme, followed by ":".

The protocol attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. Let possibleFailure be the result of basic URL parsing the given value, followed by ":", with copyURL as url and scheme start state as state override.

  4. If possibleFailure is failure, throw a TypeError exception.

  5. If copyURL’s scheme is not "http" or "https", terminate these steps.

  6. Location-object-setter navigate to copyURL.

The host attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let url be this Location object’s URL.

  3. If url’s host is null, return the empty string.

  4. If url’s port is null, return url’s host, serialized.

  5. Return url’s host, serialized, followed by ":" and url’s port, serialized.

The host attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If copyURL’s non-relative flag is set, terminate these steps.

  4. Basic URL parse the given value, with copyURL as url and host state as state override.

  5. Location-object-setter navigate to copyURL.

The hostname attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. If this Location object’s URL’s host is null, return the empty string.

  3. Return this Location object’s URL’s host, serialized.

The hostname attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If copyURL’s non-relative flag is set, terminate these steps.

  4. Basic URL parse the given value, with copyURL as url and hostname state as state override.

  5. Location-object-setter navigate to copyURL.

The port attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. If this Location object’s URL’s port is null, return the empty string.

  3. Return this Location object’s URL’s port, serialized.

The port attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If copyURL’s host is null, copyURL’s non-relative flag is set, or copyURL’s scheme is "file", terminate these steps.

  4. Basic URL parse the given value, with copyURL as url and port state as state override.

  5. Location-object-setter navigate to copyURL.

The pathname attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let url be this Location object’s URL.

  3. If url’s non-relative flag is set, return the first string in url’s path.

  4. Return "/", followed by the strings in url’s path (including empty strings), separated from each other by "/".

The pathname attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If copyURL’s non-relative flag is set, terminate these steps.

  4. Set copyURL’s path to the empty list.

  5. Basic URL parse the given value, with copyURL as url and path start state as state override.

  6. Location-object-setter navigate to copyURL.

The search attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. If this Location object’s URL’s query is either null or the empty string, return the empty string.

  3. Return "?", followed by this Location object’s URL’s query.

The search attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If the given value is the empty string, set copyURL’s query to null.

  4. Otherwise, run these substeps:

    1. Let input be the given value with a single leading "?" removed, if any.

    2. Set copyURL’s query to the empty string.

    3. Basic URL parse input, with copyURL as url and query state as state override, and the relevant Document’s document’s character encoding as encoding override.

  5. Location-object-setter navigate to copyURL.

The hash attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. If this Location object’s URL’s fragment is either null or the empty string, return the empty string.

  3. Return "#", followed by this Location object’s URL’s fragment.

The hash attribute’s setter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Let copyURL be a copy of this Location object’s URL.

  3. If copyURL’s scheme is "javascript", terminate these steps.

  4. Let input be the given value with a single leading "#" removed, if any.

  5. Set copyURL’s fragment to the empty string.

  6. Basic URL parse input, with copyURL as url and fragment state as state override.

  7. Location-object-setter navigate to copyURL.

Unlike the equivalent API for the a and area elements, the hash attribute’s setter does not special case the empty string to remain compatible with deployed scripts.


When the assign(url) method is invoked, the user agent must run the following steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Parse url, relative to the entry settings object. If that failed, throw a "SyntaxError" DOMException.

  3. Location-object navigate to the resulting URL record.

When the replace(url) method is invoked, the user agent must run the following steps:

  1. Parse url, relative to the entry settings object. If that failed, throw a "SyntaxError" DOMException.

  2. Location-object navigate to the resulting URL record with the replacement flag set.

The replace() method intentionally has no security check.

When the reload() method is invoked, the user agent must run the appropriate steps from the following list:

If this Location object’s relevant Document’s origin is not same origin with entry settings object’s origin
Throw a "SecurityError" DOMException.
If the currently executing task is the dispatch of a resize event in response to the user resizing the browsing context
Repaint the browsing context and abort these steps.
If the browsing context’s active document is an iframe srcdoc document
Reprocess the iframe attributes of the browsing context’s browsing context container.
If the browsing context’s active document has its reload override flag set
Perform an overridden reload, with the browsing context being navigated as the responsible browsing context.
Otherwise
Navigate the browsing context to the document’s URL with the exceptions enabled flag set and replacement enabled. The source browsing context must be the browsing context being navigated. This is a reload-triggered navigation. Rethrow any exceptions.

When a user requests that the active document of a browsing context be reloaded through a user interface element, the user agent should navigate the browsing context to the same resource as that Document, with replacement enabled. In the case of non-idempotent methods (e.g., HTTP POST), the user agent should prompt the user to confirm the operation first, since otherwise transactions (e.g., purchases or database modifications) could be repeated. User agents may allow the user to explicitly override any caches when reloading. If browsing context’s active document’s reload override flag is set, then the user agent may instead perform an overridden reload rather than the navigation described in this paragraph (with the browsing context being reloaded as the source browsing context).

The ancestorOrigins attribute’s getter must run these steps:

  1. If this Location object’s relevant Document’s origin is not same origin-domain with the entry settings object’s origin, then throw a "SecurityError" DOMException.

  2. Otherwise, return this Location object’s ancestor origins array.

6.6.4.1. The Location internal methods

The Location object requires additional logic beyond IDL for security purposes. The internal slot and internal methods Location objects must implement are defined below.

Every Location object has a [[DefaultProperties]] internal slot representing its own properties at time of its creation.

6.6.4.1.1. [[GetPrototypeOf]] ( )
  1. If IsPlatformObjectSameOrigin(this) is true, then return ! OrdinaryGetPrototypeOf(this).

  2. Return null.

6.6.4.1.2. [[SetPrototypeOf]] ( V )
  1. Return false.

6.6.4.1.3. [[IsExtensible]] ( )
  1. Return true.

6.6.4.1.4. [[PreventExtensions]] ( )
  1. Return false.

6.6.4.1.5. [[GetOwnProperty]] ( P )
  1. If IsPlatformObjectSameOrigin(this) is true, then:

    1. Let desc be OrdinaryGetOwnProperty(this, P).

    2. If the value of the [[DefaultProperties]] internal slot of this contains P, then set desc.[[Configurable]] to true.

    3. Return desc.

  2. Let property be ! CrossOriginGetOwnPropertyHelper(this, P).

  3. If property is not undefined, return property.

  4. Throw a "SecurityError" DOMException.

6.6.4.1.6. [[DefineOwnProperty]] ( P, Desc )
  1. If IsPlatformObjectSameOrigin(this) is true, then:

    1. If the value of the [[DefaultProperties]] internal slot of this contains P, then return false.

    2. Return ? OrdinaryDefineOwnProperty(this, P, Desc).

  2. Return false.

6.6.4.1.7. [[Get]] ( P, Receiver )
  1. If IsPlatformObjectSameOrigin(this) is true, then return ? OrdinaryGet(this, P, Receiver).

  2. Return ? CrossOriginGet(this, P, Receiver).

6.6.4.1.8. [[Set]] ( P, V, Receiver )
  1. If IsPlatformObjectSameOrigin(this) is true, then return ? OrdinarySet(this, P, Receiver).

  2. Return ? CrossOriginSet(this, P, V, Receiver).

6.6.4.1.9. [[Delete]] ( P )
  1. If IsPlatformObjectSameOrigin(this) is true, then return ? OrdinaryDelete(this, P).

  2. Return false.

6.6.4.1.10. [[OwnPropertyKeys]] ( )
  1. If IsPlatformObjectSameOrigin(this) is true, then return ! OrdinaryOwnPropertyKeys(this).

  2. Return ! CrossOriginOwnPropertyKeys(this).

6.7. Browsing the Web

Certain actions cause the browsing context to navigate to a new resource. A user agent may provide various ways for the user to explicitly cause a browsing context to navigate, in addition to those defined in this specification.

For example, following a hyperlink, §4.10.21 Form submission, and the window.open() and location.assign() methods can all cause a browsing context to navigate.

A resource has a URL, but that might not be the only information necessary to identify it. For example, a form submission that uses HTTP POST would also have the HTTP method and payload. Similarly, an iframe srcdoc document needs to know the data it is to use.

Navigation always involves source browsing context, which is the browsing context which was responsible for starting the navigation.

When a browsing context is navigated to a new resource, the user agent must run the following steps:

  1. If the source browsing context is not allowed to navigate the browsing context being navigated, then abort these steps.

    If these steps are aborted here, the user agent may instead offer to open the new resource in a new top-level browsing context or in the top-level browsing context of the source browsing context, at the user’s option, in which case the user agent must navigate that designated top-level browsing context to the new resource as if the user had requested it independently.

    Doing so, however, can be dangerous, as it means that the user is overriding the author’s explicit request to sandbox the content.

    If the navigate algorithm was invoked optionally with an exceptions enabled flag, and it is aborted on this step, then in addition to aborting this algorithm, the user agent must also throw a "SecurityError" DOMException.

  2. If there is a preexisting attempt to navigate the browsing context, and the source browsing context is the same as the browsing context being navigated, and that attempt is currently running the unload a document algorithm, and the origin of the URL of the resource being loaded in that navigation is not the same origin as the origin of the URL of the resource being loaded in this navigation, then abort these steps without affecting the preexisting attempt to navigate the browsing context.
  3. If a task queued by the traverse the history by a delta algorithm is running the unload a document algorithm for the active document of the browsing context being navigated, then abort these steps without affecting the unload a document algorithm or the aforementioned history traversal task.
  4. If the prompt to unload a document algorithm is being run for the active document of the browsing context being navigated, then abort these steps without affecting the prompt to unload a document algorithm.
  5. Let gone async be false.

    The handle redirects step later in this algorithm can in certain cases jump back to the step labeled Fragments. Since, between those two steps, this algorithm goes from operating immediately in the context of the calling task to operating in parallel independent of the event loop, some of the intervening steps need to be able to handle both being run as part of a task and running in parallel. The gone async flag is thus used to make these steps aware of which mode they are operating in.

  6. Fragments: If this is not a reload-triggered navigation: apply the URL parser algorithm to the absolute URL of the new resource and the URL of the active document of the browsing context being navigated; if all the components of the resulting parsed URLs, ignoring any fragment components, are identical, and the new resource is to be fetched using GET, and the URL record of the new resource has a fragment component that is not null (even if it is empty), then navigate to that fragment and abort these steps.
  7. If gone async is false, cancel any preexisting but not yet mature attempt to navigate the browsing context, including canceling any instances of the fetch algorithm started by those attempts. If one of those attempts has already created and initialized a new Document object, abort that Document also. (Navigation attempts that have matured already have session history entries, and are therefore handled during the update the session history with the new page algorithm, later.)
  8. If the new resource is to be handled using a mechanism that does not affect the browsing context, e.g., ignoring the navigation request altogether because the specified scheme is not one of the supported protocols, then abort these steps and proceed with that mechanism instead.
  9. If gone async is false, prompt to unload the Document object. If the user refused to allow the document to be unloaded, then abort these steps.

    If this instance of the navigation algorithm gets canceled while this step is running, the prompt to unload a document algorithm must nonetheless be run to completion.

  10. If gone async is false, abort the active document of the browsing context.
  11. If the new resource is to be handled by displaying some sort of inline content, e.g., an error message because the specified scheme is not one of the supported protocols, or an inline prompt to allow the user to select a registered handler for the given scheme, then display the inline content and abort these steps.

    In the case of a registered handler being used, the algorithm will be reinvoked with a new URL to handle the request.

  12. If the browsing context being navigated is a nested browsing context, then put it in the delaying load events mode.

    The user agent must take this nested browsing context out of the delaying load events mode when this navigation algorithm later matures, or when it terminates (whether due to having run all the steps, or being canceled, or being aborted), whichever happens first.

  13. This is the step that attempts to obtain the resource, if necessary. Jump to the first appropriate substep:

    If the resource has already been obtained (e.g., because it is being used to populate an object element’s new child browsing context)
    Skip this step. The data is already available.
    If the new resource is a URL whose scheme is javascript

    Queue a task to run these "javascript: URL" steps, associated with the active document of the browsing context being navigated:

    1. If the origin of the source browsing context is not the same origin as the origin of the active document of the browsing context being navigated, then let result be undefined, and jump to the step labeled process results below.

    2. Let urlRecord be the result of running the URL parser on the URL of the new resource.

    3. Let script source be the empty string.

    4. Append the first string of urlRecord’s path component to script source.

    5. If urlRecord’s query component is not null, then first append a U+003F QUESTION MARK character (?) to script source, and then append urlRecord’s query component to script source.

    6. If urlRecord’s fragment component is not null, then first append a U+0023 NUMBER SIGN character (#) to script source, and then append urlRecord’s fragment component to script source.

    7. Replace script source with the result of applying the percent decode algorithm to script source.

    8. Replace script source with the result of applying the UTF-8 decode algorithm to script source.

    9. Let address be the URL of the active document of the browsing context being navigated.

    10. Let settings be the relevant settings object of the browsing context being navigated.

    11. Let script be the result of creating a classic script given script source and settings.

    12. Let result be the result of running the classic script script. If evaluation was unsuccessful, let result be undefined instead. (The result will also be undefined if scripting is disabled.)

    13. Process results: If Type(result) is not String, then the result of obtaining the resource for the URL is a response whose status is 204.

      Otherwise, the result of obtaining the resource for the URL is a response whose header list consists of Content-Type/text/html and whose body is result, and whose HTTPS state is settings’s HTTPS state.

      When it comes time to set the document’s address in the navigation algorithm, use address as the override URL.

    The task source for this task is the DOM manipulation task source.

    So for example a javascript: URL in an href attribute of an a element would only be evaluated when the link was followed, while such a URL in the src attribute of an iframe element would be evaluated in the context of the iframe’s own nested browsing context when the iframe is being set up; once evaluated, its return value (if it was not void) would replace that browsing context’s Document, thus also changing the Window object of that browsing context.
    Otherwise
    1. Let request be the new resource.
    2. If request is a URL, set request to a new request whose URL is request.
    3. Set request’s client to the source browsing context’s active document’s Window object’s environment settings object, target browsing context to the browsing context being navigated, destination to "document", mode to "navigate", credentials mode to "include", use-URL-credentials flag, and redirect mode to "manual".
    4. Set request’s omit-Origin-header flag.
    5. If request’s method is not GET, or, if the navigation algorithm was invoked as a result of the form submission algorithm, then if there is an origin of the active document of the source browsing context, unset request’s omit-Origin-header flag.
    6. Otherwise, if the browsing context being navigated is a child browsing context, and the browsing context container of the browsing context being navigated has a browsing context scope origin, set request’s origin to that browsing context scope origin and unset request’s omit-Origin-header flag.
    7. Fetch request.
  14. If gone async is false, return to whatever algorithm invoked the navigation steps and continue running these steps in parallel.

  15. Let gone async be true.
  16. Wait for one or more bytes to be available or for the user agent to establish that the resource in question is empty. During this time, the user agent may allow the user to cancel this navigation attempt or start other navigation attempts.
  17. Handle redirects: If fetching the resource results in a redirect, and either the URL of the target of the redirect has the same origin as the original resource, or the resource is being obtained using the POST method or a safe method (in HTTP terms), return to the step labeled Fragments with the new resource, except that if the URL of the target of the redirect does not have a fragment and the URL of the resource that led to the redirect does, then the fragment of the resource that led to the redirect must be propagated to the URL of the target of the redirect.

    So for instance, if the original URL was "https://example.com/#!sample" and "https://example.com/" is found to redirect to "https://example.com/", the URL of the new resource will be "https://example.com/#!sample".

    Otherwise, if fetching the resource results in a redirect but the URL of the target of the redirect does not have the same origin as the original resource and the resource is being obtained using a method that is neither the POST method nor a safe method (in HTTP terms), then abort these steps. The user agent may indicate to the user that the navigation has been aborted for security reasons.

  18. Resource handling: If the resource’s out-of-band metadata (e.g., HTTP headers), not counting any type information (such as the Content-Type HTTP header), requires some sort of processing that will not affect the browsing context, then perform that processing and abort these steps.

    Such processing might be triggered by, amongst other things, the following:
    • HTTP status codes (e.g., 204 No Content or 205 Reset Content)
    • Network errors (e.g., the network interface being unavailable)
    • Cryptographic protocol failures (e.g., an incorrect TLS certificate)

    Responses with HTTP Content-Disposition headers specifying the attachment disposition type must be handled as a download.

    HTTP 401 responses that do not include a challenge recognized by the user agent must be processed as if they had no challenge, e.g., rendering the entity body as if the response had been 200 OK.

    User agents may show the entity body of an HTTP 401 response even when the response does include a recognized challenge, with the option to login being included in a non-modal fashion, to enable the information provided by the server to be used by the user before authenticating. Similarly, user agents should allow the user to authenticate (in a non-modal fashion) against authentication challenges included in other responses such as HTTP 200 OK responses, effectively allowing resources to present HTTP login forms without requiring their use.

  19. Let type be the computed type of the resource.
  20. If the user agent has been configured to process resources of the given type using some mechanism other than rendering the content in a browsing context, then skip this step. Otherwise, if the type is one of the following types, jump to the appropriate entry in the following list, and process the resource as described there:
    an HTML MIME type
    Follow the steps given in the HTML document section, and then, once they have completed, abort this navigate algorithm.
    an XML MIME type that is not an explicitly supported XML type
    Follow the steps given in the XML document section. If that section determines that the content is not to be displayed as a generic XML document, then proceed to the next step in this overall set of steps. Otherwise, once the steps given in the XML document section have completed, abort this navigate algorithm.
    a JavaScript MIME type
    a JSON MIME type that is not an explicitly supported JSON type
    "text/css"
    "text/plain"
    "text/vtt"
    Follow the steps given in the plain text file section, and then, once they have completed, abort this navigate algorithm.
    "multipart/x-mixed-replace"
    Follow the steps given in the §12.2 multipart/x-mixed-replace section, and then, once they have completed, abort this navigate algorithm.
    A supported image, video, or audio type
    Follow the steps given in the media section, and then, once they have completed, abort this navigate algorithm.
    A type that will use an external application to render the content in the browsing context
    Follow the steps given in the plugin section, and then, once they have completed, abort this navigate algorithm.

    An explicitly supported XML type is one for which the user agent is configured to use an external application to render the content (either a plugin rendering directly in the browsing context, or a separate application), or one for which the user agent has dedicated processing rules (e.g., a Web browser with a built-in Atom feed viewer would be said to explicitly support the application/atom+xml MIME type), or one for which the user agent has a dedicated handler (e.g., one registered using registerContentHandler()).

    The term JSON MIME type is used to refer to the MIME types application/json, text/json, and any MIME type whose subtype ends with the five characters "+json".

    An explicitly supported JSON type is one for which the user agent is configured to use an external application to render the content (either a plugin rendering directly in the browsing context, or a separate application), or one for which the user agent has dedicated processing rules, or one for which the user agent has a dedicated handler (e.g., one registered using registerContentHandler()).

    Setting the document’s address: If there is no override URL, then any Document created by these steps must have its URL set to the URL that was originally to be fetched, ignoring any other data that was used to obtain the resource. However, if there is an override URL, then any Document created by these steps must have its URL set to that URL instead.

    An override URL is set when dereferencing a javascript: URL and when performing an overridden reload.

    Initializing a new Document object: when a Document is created as part of the above steps, the user agent will be required to additionally run the following algorithm after creating the new object:

    1. If browsingContext’s only entry in its session history is the about:blank Document that was added when browsingContext was created, and navigation is occurring with replacement enabled, and that Document has the same origin as the new Document, then

      1. Let window be the Window object of that Document.

      2. Change the document attribute of window to point to the new Document.

    2. Otherwise,

      1. Call the JavaScript InitializeHostDefinedRealm() abstract operation with the following customizations:

        • For the global object, create a new Window object window.

        • For the global this value, use browsingContext’s WindowProxy object.

        • Let realm execution context be the created JavaScript execution context.

        • Do not obtain any source texts for scripts or modules.

      2. Set up a browsing context environment settings object with realm execution context, and let settings object be the result.

      3. Set window’s associated Document to the new Document.

    3. Set browsingContext’s WindowProxy object’s [[Window]] internal slot value to window.

    4. Set the Document's HTTPS state to the HTTPS state of the resource used to generate the document.

    5. Execute the Initialize a Document’s CSP list algorithm on the Document object and the resource used to generate the document. [CSP3]

    6. Set the document’s referrer to the address of the resource from which Request-URIs are obtained as determined when the fetch algorithm obtained the resource, if that algorithm was used and determined such a value; otherwise, set it to the empty string.

    7. Implement the sandboxing for the Document.

    8. If the active sandboxing flag set of the Document's browsing context or any of its ancestor browsing contexts (if any) have the sandboxed fullscreen browsing context flag set, then skip this step.

      If the Document's browsing context has a browsing context container and either it is not an iframe element, or it does not have the allowfullscreen attribute specified, or its Document does not have the fullscreen enabled flag set, then also skip this step.

      Otherwise, set the Document's fullscreen enabled flag.

    9. Non-document content: If, given type, the new resource is to be handled by displaying some sort of inline content, e.g., a native rendering of the content, an error message because the specified type is not supported, or an inline prompt to allow the user to select a registered handler for the given type, then display the inline content, and then abort these steps.

      In the case of a registered handler being used, the algorithm will be reinvoked with a new URL to handle the request.

    10. Otherwise, the document’s type is such that the resource will not affect the browsing context, e.g., because the resource is to be handed to an external application or because it is an unknown type that will be processed as a download. Process the resource appropriately.

When a resource is handled by passing its URL or data to an external software package separate from the user agent (e.g., handing a mailto: URL to a mail client, or a Word document to a word processor), user agents should attempt to mitigate the risk that this is an attempt to exploit the target software, e.g., by prompting the user to confirm that the source browsing context’s active document’s origin is to be allowed to invoke the specified software. In particular, if the navigate algorithm, when it was invoked, was not allowed to show a popup, the user agent should not invoke the external software package without prior user confirmation.

For example, there could be a vulnerability in the target software’s URL handler which a hostile page would attempt to exploit by tricking a user into clicking a link.


Some of the sections below, to which the above algorithm defers in certain cases, require the user agent to update the session history with the new page. When a user agent is required to do this, it must queue a task (associated with the Document object of the current entry, not the new one) to run the following steps:

  1. Unload the Document object of the current entry, with the recycle parameter set to false.

    If this instance of the navigation algorithm is canceled while this step is running the unload a document algorithm, then the unload a document algorithm must be allowed to run to completion, but this instance of the navigation algorithm must not run beyond this step. (In particular, for instance, the cancelation of this algorithm does not abort any event dispatch or script execution occurring as part of unloading the document or its descendants.)

  2. If the navigation was initiated for entry update of an entry
    1. Replace the Document of the entry being updated, and any other entries that referenced the same document as that entry, with the new Document.
    2. Traverse the history to the new entry.

    This can only happen if the entry being updated is not the current entry, and can never happen with replacement enabled. (It happens when the user tried to traverse to a session history entry that no longer had a Document object.)

    Otherwise
    1. Remove all the entries in the browsing context’s session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

      This doesn’t necessarily have to affect the user agent’s user interface.

    2. Append a new entry at the end of the History object representing the new resource and its Document object, related state, and the default scroll restoration mode of "auto".
    3. Traverse the history to the new entry. If the navigation was initiated with replacement enabled, then the traversal must itself be initiated with replacement enabled.
  3. The navigation algorithm has now matured.
  4. fragment loop: Spin the event loop for a user-agent-defined amount of time, as desired by the user agent implementor. (This is intended to allow the user agent to optimize the user experience in the face of performance concerns.)
  5. If the Document object has no parser, or its parser has stopped parsing, or the user agent has reason to believe the user is no longer interested in scrolling to the fragment, then abort these steps.
  6. Scroll to the fragment given in the document’s URL. If this fails to find an indicated part of the document, then return to the fragment loop step.

The task source for this task is the networking task source.

6.7.2. Page load processing model for HTML files

When an HTML document is to be loaded in a browsing context, the user agent must queue a task to create a Document object, mark it as being an HTML document, set its content type to "text/html", initialize the Document object, and finally create an HTML parser and associate it with the Document. Each task that the networking task source places on the task queue while fetching runs must then fill the parser’s input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

The input byte stream converts bytes into characters for use in the tokenizer. This process relies, in part, on character encoding information found in the real Content-Type metadata of the resource; the "computed type" is not used for this purpose.

When no more bytes are available, the user agent must queue a task for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but before any script execution, certainly before the parser stops, the user agent must update the session history with the new page.

The task source for the two tasks mentioned in this section must be the networking task source.

6.7.3. Page load processing model for XML files

When faced with displaying an XML file inline, user agents must follow the requirements defined in the XML and Namespaces in XML recommendations, RFC 7303, DOM, and other relevant specifications to create a Document object and a corresponding XML parser. [XML] [XML-NAMES] [RFC7303] [DOM]

At the time of writing, the XML specification community had not actually yet specified how XML and the DOM interact.

After the Document is created, the user agent must initialize the Document object.

The actual HTTP headers and other metadata, not the headers as mutated or implied by the algorithms given in this specification, are the ones that must be used when determining the character encoding according to the rules given in the above specifications. Once the character encoding is established, the document’s character encoding must be set to that character encoding.

User agents may examine the namespace of the root Element node of this Document object to perform namespace-based dispatch to alternative processing tools, e.g., determining that the content is actually a syndication feed and passing it to a feed handler. If such processing is to take place, abort the steps in this section, and jump to the next step (labeled non-document content) in the navigate steps above.

Otherwise, then, with the newly created Document, the user agent must update the session history with the new page. User agents may do this before the complete document has been parsed (thus achieving incremental rendering), and must do this before any scripts are to be executed.

Error messages from the parse process (e.g., XML namespace well-formedness errors) may be reported inline by mutating the Document.

6.7.4. Page load processing model for text files

When a plain text document is to be loaded in a browsing context, the user agent must queue a task to create a Document object, mark it as being an HTML document, set its content type to the computed MIME type of the resource (type in the navigate algorithm), initialize the Document object, create an HTML parser, associate it with the Document, act as if the tokenizer had emitted a start tag token with the tag name "pre" followed by a single U+000A LINE FEED (LF) character, and switch the HTML parser’s tokenizer to the §8.2.4.7 PLAINTEXT state. Each task that the networking task source places on the task queue while fetching runs must then fill the parser’s input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

The rules for how to convert the bytes of the plain text document into actual characters, and the rules for actually rendering the text to the user, are defined by the specifications for the computed MIME type of the resource (type in the navigate algorithm).

The document’s character encoding must be set to the character encoding used to decode the document.

When no more bytes are available, the user agent must queue a task for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but potentially before the page has finished parsing, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, e.g., to link to a style sheet, provide a script, give the document a title, etc.

In particular, if the user agent supports the Format=Flowed feature of RFC3676 then the user agent would need to apply extra styling to cause the text to wrap correctly and to handle the quoting feature. [RFC3676]

The task source for the two tasks mentioned in this section must be the networking task source.

6.7.5. Page load processing model for multipart/x-mixed-replace resources

When a resource with the type multipart/x-mixed-replace is to be loaded in a browsing context, the user agent must parse the resource using the rules for multipart types. [RFC2046]

For each body part obtained from the resource, the user agent must run a new instance of the navigate algorithm, starting from the resource handling step, using the new body part as the resource being navigated, with replacement enabled if a previous body part from the same resource resulted in a Document object being created and initialized, and otherwise using the same setup as the navigate attempt that caused this section to be invoked in the first place.

For the purposes of algorithms processing these body parts as if they were complete stand-alone resources, the user agent must act as if there were no more bytes for those resources whenever the boundary following the body part is reached.

Thus, load events (and for that matter unload events) do fire for each body part loaded.

6.7.6. Page load processing model for media

When an image, video, or audio resource is to be loaded in a browsing context, the user agent should create a Document object, mark it as being an HTML document, set its content type to the computed MIME type of the resource (type in the navigate algorithm), initialize the Document object, append an html element to the Document, append a head element and a body element to the html element, append an element host element for the media, as described below, to the body element, and set the appropriate attribute of the element host element, as described below, to the address of the image, video, or audio resource.

The element host element to create for the media is the element given in the table below in the second cell of the row whose first cell describes the media. The appropriate attribute to set is the one given by the third cell in that same row.

Type of media Element for the media Appropriate attribute
Image img src
Video video src
Audio audio src

Then, the user agent must act as if it had stopped parsing.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, or attributes to the element host element, e.g., to link to a style sheet, provide a script, give the document a title, make the media autoplay, etc.

6.7.7. Page load processing model for content that uses plugins

When a resource that requires an external resource to be rendered is to be loaded in a browsing context, the user agent should create a Document object, mark it as being an HTML document and mark it as being a plugin document, set its content type to the computed MIME type of the resource (type in the navigate algorithm), initialize the Document object, append an html element to the Document, append a head element and a body element to the html element, append an embed to the body element, and set the src attribute of the embed element to the address of the resource.

The term plugin document is used by Content Security Policy as part of the mechanism that ensures iframes can’t be used to evade plugin-types directives. [CSP3]

Then, the user agent must act as if it had stopped parsing.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, or attributes to the embed element, e.g., to link to a style sheet, to give the document a title, etc.

If the Document's active sandboxing flag set has its sandboxed plugins browsing context flag set, the synthesized embed element will fail to render the content if the relevant plugin cannot be secured.

6.7.8. Page load processing model for inline content that doesn’t have a DOM

When the user agent is to display a user agent page inline in a browsing context, the user agent should create a Document object, mark it as being an HTML document, set its content type to "text/html", initialize the Document object, and then either associate that Document with a custom rendering that is not rendered using the normal Document rendering rules, or mutate that Document until it represents the content the user agent wants to render.

Once the page has been set up, the user agent must act as if it had stopped parsing.

After creating the Document object, but potentially before the page has been completely set up, the user agent must update the session history with the new page.

When a user agent is supposed to navigate to a fragment, then the user agent must run the following steps:

  1. Remove all the entries in the browsing context’s session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

    This doesn’t necessarily have to affect the user agent’s user interface.

  2. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context’s document family.
  3. Append a new entry at the end of the History object representing the new resource and its Document object, related state, and current history scroll restoration preference. Its URL must be set to the address to which the user agent was navigating. The title must be left unset.
  4. Traverse the history to the new entry, with the non-blocking events flag set. This will scroll to the fragment given in what is now the document’s URL.

If the scrolling fails because the relevant ID has not yet been parsed, then the original navigation algorithm will take care of the scrolling instead, as the last few steps of its update the session history with the new page algorithm.


When the user agent is required to scroll to the fragment and the indicated part of the document, if any, is being rendered, the user agent must either change the scrolling position of the document using the following algorithm, or perform some other action such that the indicated part of the document is brought to the user’s attention. If there is no indicated part, or if the indicated part is not being rendered, then the user agent must do nothing. The aforementioned algorithm is as follows:

  1. Let target be the indicated part of the document, as defined below.
  2. If target is the top of the document, then scroll to the beginning of the document for the Document, and abort these steps. [CSSOM-VIEW]
  3. Use the scroll an element into view algorithm to scroll target into view, with the align to top flag set. [CSSOM-VIEW]
  4. Run the focusing steps for that element, with the Document's viewport as the fallback target.
  5. Move the sequential focus navigation starting point to target.

The indicated part of the document is the one that the fragment, if any, identifies. The semantics of the fragment in terms of mapping it to a specific DOM Node is defined by the specification that defines the MIME type used by the Document (for example, the processing of fragment for XML MIME types is the responsibility of RFC7303). [RFC7303]

For HTML documents (and HTML MIME types), the following processing model must be followed to determine what the indicated part of the document is.

  1. Apply the URL parser algorithm to the URL, and let fragid be the fragment component of the resulting URL record.
  2. If fragid is the empty string, then the indicated part of the document is the top of the document; stop the algorithm here.
  3. Let fragid bytes be the result of percent decoding fragid.
  4. Let decoded fragid be the result of running UTF-8 decode without BOM or fail on fragid bytes. If decoded fragid is failure, jump to the step labeled no decoded fragid.
  5. If there is an element in the DOM that has an ID exactly equal to decoded fragid, then the first such element in tree order is the indicated part of the document; stop the algorithm here.
  6. No decoded fragid: If there is an a element in the DOM that has a name attribute whose value is exactly equal to fragid (not decoded fragid), then the first such element in tree order is the indicated part of the document; stop the algorithm here.
  7. If fragid is an ASCII case-insensitive match for the string top, then the indicated part of the document is the top of the document; stop the algorithm here.
  8. Otherwise, there is no indicated part of the document.

For the purposes of the interaction of HTML with Selectors' :target pseudo-class, the target element is the indicated part of the document, if that is an element; otherwise there is no target element. [SELECTORS4]

The task source for the task mentioned in this section must be the DOM manipulation task source.

6.7.10. History traversal

When a user agent is required to traverse the history to a specified entry, optionally with replacement enabled, and optionally with the non-blocking events flag set, the user agent must act as follows.

This algorithm is not just invoked when explicitly going back or forwards in the session history — it is also invoked in other situations, for example when navigating a browsing context, as part of updating the session history with the new page.

  1. If there is no longer a Document object for the entry in question, navigate the browsing context to the resource for that entry to perform an entry update of that entry, and abort these steps. The "navigate" algorithm reinvokes this "traverse" algorithm to complete the traversal, at which point there is a Document object and so this step gets skipped. The navigation must be done using the same source browsing context as was used the first time this entry was created. (This can never happen with replacement enabled.)

    If the resource was obtained using a non-idempotent action, for example a POST form submission, or if the resource is no longer available, for example because the computer is now offline and the page wasn’t cached, navigating to it again might not be possible. In this case, the navigation will result in a different page than previously; for example, it might be an error message explaining the problem or offering to resubmit the form.

  2. If the current entry’s title was not set by the pushState() or replaceState() methods, then set its title to the value returned by the document.title IDL attribute.
  3. If appropriate, update the current entry in the browsing context’s Document object’s History object to reflect any state that the user agent wishes to persist. The entry is then said to be an entry with persisted user state.
  4. If the specified entry has a different Document object than the current entry, then run the following substeps:
    1. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context’s document family.
    2. If the origin of the Document of the specified entry is not the same as the origin of the Document of the current entry, then run the following sub-sub-steps:

      1. The current browsing context name must be stored with all the entries in the history that are associated with Document objects with the same origin as the active document and that are contiguous with the current entry.
      2. If the browsing context is a top-level browsing context, but not an auxiliary browsing context, then the browsing context’s browsing context name must be unset.
    3. Make the specified entry’s Document object the active document of the browsing context.
    4. If the specified entry has a browsing context name stored with it, then run the following sub-sub-steps:

      1. Set the browsing context’s browsing context name to the name stored with the specified entry.
      2. Clear any browsing context names stored with all entries in the history that are associated with Document objects with the same origin as the new active document and that are contiguous with the specified entry.
    5. If the specified entry’s Document has any form controls whose autofill field name is "off", invoke the reset algorithm of each of those elements.
    6. If the current document readiness of the specified entry’s Document is "complete", queue a task to run the following sub-sub-steps:

      1. If the Document's page showing flag is true, then abort this task (i.e., don’t fire the event below).
      2. Set the Document's page showing flag to true.
      3. Run any session history document visibility change steps for Document that are defined by other applicable specifications.

        This is specifically intended for use by the Page Visibility specification. [PAGE-VISIBILITY]

      4. Fire a trusted event with the name pageshow at the Window object of that Document, with target override set to the Document object, using the PageTransitionEvent interface, with the persisted attribute initialized to true. This event must not bubble, must not be cancelable, and has no default action.
  5. Set the document’s URL to the URL of the specified entry.
  6. If the specified entry has a URL whose fragment differs from that of the current entry’s when compared in a case-sensitive manner, and the two share the same Document object, then let hash changed be true, and let old URL be the URL of the current entry and new URL be the URL of the specified entry. Otherwise, let hash changed be false.
  7. If the traversal was initiated with replacement enabled, remove the entry immediately before the specified entry in the session history.
  8. If the specified entry is not an entry with persisted user state, but its URL has a fragment, scroll to the fragment.
  9. If the entry is an entry with persisted user state, the user agent may restore persisted user state and update aspects of the document and its rendering.
  10. Let targetRealm be the current Realm Record.
  11. If the entry is a state object entry, let state be StructuredClone(that state object, targetRealm) of that state object. Otherwise, let state be null.
  12. Set history.state to state.
  13. Let state changed be true if the Document of the specified entry has a latest entry, and that entry is not the specified entry; otherwise let it be false.
  14. Let the latest entry of the Document of the specified entry be the specified entry.
  15. If the non-blocking events flag is not set, then run the following steps immediately. Otherwise, the non-blocking events flag is set; queue a task to run the following substeps instead.

    1. If state changed is true, fire a trusted event with the name popstate at the Window object of the Document, using the PopStateEvent interface, with the state attribute initialized to the value of state. This event must bubble but not be cancelable and has no default action.
    2. If hash changed is true, then fire a trusted event with the name hashchange at the browsing context’s Window object, using the HashChangeEvent interface, with the oldURL attribute initialized to old URL and the newURL attribute initialized to new URL. This event must bubble but not be cancelable and has no default action.
  16. The current entry is now the specified entry.

The task source for the tasks mentioned above is the DOM manipulation task source.

6.7.10.1. Persisted user state restoration

When the user agent is to restore persisted user state from a history entry, it must run the following steps immediately:

  1. If the entry has a scroll restoration mode, let scrollRestoration be that. Otherwise let scrollRestoration be "auto"
  2. If scrollRestoration is "manual" the user agent should not restore the scroll position for the document, otherwise, it may do so.
  3. Optionally, update other aspects of the document and its rendering, for instance values of form fields, that the user agent had previously recorded.

This can even include updating the dir attribute of textarea elements or input elements whose type attribute is in either the Text state or the Search state, if the persisted state includes the directionality of user input in such controls.

6.7.10.2. The PopStateEvent interface
[Constructor(DOMString type, optional PopStateEventInit eventInitDict), Exposed=(Window,Worker)]
interface PopStateEvent : Event {
  readonly attribute any state;
};
dictionary PopStateEventInit : EventInit {
  any state = null;
};
event . state
Returns a copy of the information that was provided to pushState() or replaceState().

The state attribute must return the value it was initialized to. It represents the context information for the event, or null, if the state represented is the initial state of the Document.

6.7.10.3. The HashChangeEvent interface
[Constructor(DOMString type, optional HashChangeEventInit eventInitDict), Exposed=(Window,Worker)]
interface HashChangeEvent : Event {
  readonly attribute USVString oldURL;
  readonly attribute USVString newURL;
};
dictionary HashChangeEventInit : EventInit {
  USVString oldURL = "";
  USVString newURL = "";
};
event . oldURL
Returns the URL of the session history entry that was previously current.
event . newURL
Returns the URL of the session history entry that is now current.

The oldURL attribute must return the value it was initialized to. It represents context information for the event, specifically the URL of the session history entry that was traversed from.

The newURL attribute must return the value it was initialized to. It represents context information for the event, specifically the URL of the session history entry that was traversed to.

6.7.10.4. The PageTransitionEvent interface
[Constructor(DOMString type, optional PageTransitionEventInit eventInitDict), Exposed=(Window,Worker)]
interface PageTransitionEvent : Event {
  readonly attribute boolean persisted;
};
dictionary PageTransitionEventInit : EventInit {
  boolean persisted = false;
};
event . persisted
For the pageshow event, returns false if the page is newly being loaded (and the load event will fire). Otherwise, returns true.

For the pagehide event, returns false if the page is going away for the last time. Otherwise, returns true, meaning that (if nothing conspires to make the page unsalvageable) the page might be reused if the user navigates back to this page.

Things that can cause the page to be unsalvageable include:

  • document.open()
  • Listening for beforeunload events
  • Listening for unload events
  • Having iframes that are not salvageable
  • Active WebSocket objects
  • Aborting a Document

The persisted attribute must return the value it was initialized to. It represents the context information for the event.

6.7.11. Unloading documents

A Document has a salvageable state, which must initially be true, a fired unload flag, which must initially be false, and a page showing flag, which must initially be false. The page showing flag is used to ensure that scripts receive pageshow and pagehide events in a consistent manner (e.g., that they never receive two pagehide events in a row without an intervening pageshow, or vice versa).

Event loops have a termination nesting level counter, which must initially be zero.

When a user agent is to prompt to unload a document, it must run the following steps.

  1. Increase the event loop’s termination nesting level by one.
  2. Increase the Document's ignore-opens-during-unload counter by one.
  3. Let event be a new trusted BeforeUnloadEvent event object with the name beforeunload, which does not bubble but is cancelable.
  4. Dispatch: Dispatch event at the Document's Window object.
  5. Decrease the event loop’s termination nesting level by one.
  6. If any event listeners were triggered by the earlier dispatch step, then set the Document's salvageable state to false.
  7. If the Document's active sandboxing flag set does not have its sandboxed modals flag set, and the returnValue attribute of the event object is not the empty string, or if the event was canceled, then the user agent should ask the user to confirm that they wish to unload the document.

    The prompt shown by the user agent may include the string of the returnValue attribute, or optionally truncated.

    The user agent must pause while waiting for the user’s response.

    If the user did not confirm the page navigation, then the user agent refused to allow the document to be unloaded.

  8. If this algorithm was invoked by another instance of the "prompt to unload a document" algorithm (i.e., through the steps below that invoke this algorithm for all descendant browsing contexts), then jump to the step labeled end.
  9. Let descendants be the list of the descendant browsing contexts of the Document.
  10. If descendants is not an empty list, then for each browsing context b in descendants run the following substeps:

    1. Prompt to unload the active document of the browsing context b. If the user refused to allow the document to be unloaded, then the user implicitly also refused to allow this document to be unloaded; jump to the step labeled end.
    2. If the salvageable state of the active document of the browsing context b is false, then set the salvageable state of this document to false also.
  11. End: Decrease the Document's ignore-opens-during-unload counter by one.

When a user agent is to unload a document, it must run the following steps. These steps are passed an argument, recycle, which is either true or false, indicating whether the Document object is going to be re-used. (This is set by the document.open() method.)

  1. Increase the event loop’s termination nesting level by one.
  2. Increase the Document's ignore-opens-during-unload counter by one.
  3. If the Document's page showing flag is false, then jump to the step labeled unload event below (i.e., skip firing the pagehide event and don’t rerun the unloading document visibility change steps).
  4. Set the Document's page showing flag to false.
  5. Fire a trusted event with the name pagehide at the Window object of the Document, with target override set to the Document object, using the PageTransitionEvent interface, with the persisted attribute initialized to true if the Document object’s salvageable state is true, and false otherwise. This event must not bubble, must not be cancelable, and has no default action.
  6. Run any unloading document visibility change steps for Document that are defined by other applicable specifications.

    This is specifically intended for use by the Page Visibility specification. [PAGE-VISIBILITY]

  7. Unload event: If the Document's fired unload flag is false, fire a simple event named unload at the Document's Window object, with target override set to the Document object.
  8. Decrease the event loop’s termination nesting level by one.
  9. If any event listeners were triggered by the earlier unload event step, then set the Document object’s salvageable state to false and set the Document's fired unload flag to true.
  10. Run any unloading document cleanup steps for Document that are defined by this specification and other applicable specifications.
  11. If this algorithm was invoked by another instance of the "unload a document" algorithm (i.e., by the steps below that invoke this algorithm for all descendant browsing contexts), then jump to the step labeled end.
  12. Let descendants be the list of the descendant browsing contexts of the Document.
  13. If descendants is not an empty list, then for each browsing context b in descendants run the following substeps:

    1. Unload the active document of the browsing context b with the recycle parameter set to false.
    2. If the salvageable state of the active document of the browsing context b is false, then set the salvageable state of this document to false also.
  14. If both the Document's salvageable state and recycle are false, then the Document's browsing context must discard the Document.
  15. End: Decrease the Document's ignore-opens-during-unload counter by one.

This specification defines the following unloading document cleanup steps. Other specifications can define more.

  1. Make disappear any WebSocket objects that were created by the WebSocket() constructor from the Document's Window object.

    If this affected any WebSocket objects, then set Document's salvageable state to false.

  2. If the Document's salvageable state is false, forcibly close any EventSource objects that whose constructor was invoked from the Document's Window object.
  3. If the Document's salvageable state is false, empty the Document's Window’s list of active timers.
6.7.11.1. The BeforeUnloadEvent interface
interface BeforeUnloadEvent : Event {
  attribute DOMString returnValue;
};
event . returnValue [ = value ]

Returns the current return value of the event (the message to show the user).

Can be set, to update the message.

There are no BeforeUnloadEvent-specific initialization methods.

The returnValue attribute represents the message to show the user. When the event is created, the attribute must be set to the empty string. On getting, it must return the last value it was set to. On setting, the attribute must be set to the new value.

6.7.12. Aborting a document load

If a Document is aborted, the user agent must run the following steps:

  1. Abort the active documents of every child browsing context. If this results in any of those Document objects having their salvageable state set to false, then set this Document's salvageable state to false also.
  2. Cancel any instances of the fetch algorithm in the context of this Document, discarding any tasks queued for them, and discarding any further data received from the network for them. If this resulted in any instances of the fetch algorithm being canceled or any queued tasks or any network data getting discarded, then set the Document's salvageable state to false.
  3. If the Document has an active parser, then abort that parser and set the Document's salvageable state to false.

User agents may allow users to explicitly invoke the abort a document algorithm for a Document. If the user does so, then, if that Document is an active document, the user agent should queue a task to fire a simple event named abort at that Document's Window object before invoking the abort algorithm.

6.7.13. Browser state

[NoInterfaceObject, Exposed=(Window, Worker)]
interface NavigatorOnLine {
  readonly attribute boolean onLine;
};
window . navigator . onLine

Returns false if the user agent is definitely offline (disconnected from the network). Returns true if the user agent might be online.

The events online and offline are fired when the value of this attribute changes.

The navigator.onLine attribute must return false if the user agent will not contact the network when the user follows links or when a script requests a remote page (or knows that such an attempt would fail), and must return true otherwise.

When the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from true to false, the user agent must queue a task to fire a simple event named offline at the Window or WorkerGlobalScope object.

On the other hand, when the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from false to true, the user agent must queue a task to fire a simple event named online at the Window or WorkerGlobalScope object.

The task source for these tasks is the networking task source.

This attribute is inherently unreliable. A computer can be connected to a network without having Internet access.

In this example, an indicator is updated as the browser goes online and offline.
<!DOCTYPE HTML>
<html>
  <head>
    <title>Online status</title>
    <script>
      function updateIndicator() {
        document.getElementById('indicator').textContent = navigator.onLine ? 'online' : 'offline';
      }
    </script>
  </head>
  <body onload="updateIndicator()" ononline="updateIndicator()" onoffline="updateIndicator()">
    <p>The network is: <span>(state unknown)</span>
  </body>
</html>