CSS View Transitions Module Level 1

W3C Working Draft,

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https://www.w3.org/TR/2022/WD-css-view-transitions-1-20221124/
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Khushal Sagar (Google)
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Abstract

This module defines the View Transition API, along with associated properties and pseudo-elements.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the CSS Working Group as a Working Draft using the Recommendation track. Publication as a Working Draft does not imply endorsement by W3C and its Members.

This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-view-transitions” in the title, like this: “[css-view-transitions] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 2 November 2021 W3C Process Document.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

1. Introduction

This section is non-normative.

View Transitions is a set of API that allow DOM changes to smoothly animate between states. This is accomplished by leveraging user-agents ability to persist visual representations of state (i.e. snapshots) and blend them with current DOM state’s visual output. The API also allows the animations to be customized via standard CSS animation properties.

This spec describes the CSS and JS mechanics of the single-page transition API.

2. Transitions as an enhancement

This section is non-normative.

A key part of this API design is the view that an animated transition is an enhancement to a DOM change. Specifically, there are two components of the API: the DOM change, and the visual state animation.

In order for the user-agent to generate a set of snapshots prior to the DOM change, the API is designed to take the DOM change callback as part of the request to initiate a transition. This means that the user-agent has an opportunity to generate snapshots for the existing visual representation, run the given callback to update the state, and finally generate structures and set up animations required for the transition to occur. All this is accomplished with a single call to the JavaScript API.

The user-agent provides additional JavaScript functionality and promises to control and observe the state of the animations. These are described in detail in the ViewTransition section.

After the callback has finished running, the user-agent creates a structure of pseudo-elements that represent both the "old" and "new" states of the transition. The structure of the pseudo-elements is dictated by the view-transition-name property. Specifically, each element that is named with a view-transition-name creates a separate snapshot and thus a separate group of pseudo-elements that are animated independently from the rest. Note that for convenience, the document element is given the view-transition-name "root" in the user-agent style sheet.

The groups induced by the view-transition-name are all positioned within a common pseudo-element root, which itself is attached to the document element of the page.

Each of the groups is comprised of up to four pseudo elements:

Group

This element initially mirrors the size and position of the "old" element that it represents (i.e. the transition-named element that caused this group to be created). The element is animated to the "new" size and position.

Image-Pair

This element is a child of the group element and provides isolation: isolate for its children. It’s needed so that its children can be blended with non-normal blend modes without affecting other visual outputs.

Old image

This element is a child of the image-pair element. It is a replaced element that produced the visual representation of the "old" state taken from user-agent provided snapshots. Note that the contents of this element can be manipulated with object-* properties in the same way that other replaced elements can be.

New image

This element is a child of the image-pair element. It is a replaced element that produces the visual representation of the "new" state, taken from the page’s visual output of the represented elements. Like the old image, the contents can be manipulated with object-* properties. Note that because this element’s snapshots are taken from the pages output, the visual output changes if the visual output of the represented element changes. This is similar to element(), although it differs in terms of the captured area.

This sections can benefit from diagrams.

Each of the pseudo-elements generated can be targeted by CSS in order to customize its appearance, behavior and/or add animations. This enables full customization of the transition.

Note that because these APIs are an enhancement to the DOM change, some principles emerge:

3. CSS properties

3.1. view-transition-name

Name: view-transition-name
Value: none | <custom-ident>
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete

The view-transition-name property "names" an element as participating in a view transition.

none

The element will not participate in a view transition.

<custom-ident>

The element can participate in a view transition, as either an old or new element, with a view transition name equal to the <custom-ident>'s value.

Note: The value none is invalid as a <custom-ident>.

The document element participates in a view transition by default using the following rule, added to the HTML user agent style sheet:

html {
	view-transition-name: root;
}

Note: This property causes the user-agent to both capture separate snapshots from the elements, as well as create separate pseudo-element sub-trees representing this element’s "old" and "new" states. For the purposes of this API, if one element has a transition-name "foo" in the old state, and another element has a transition-name "foo" in the new state, they are treated as representing different visual state of the same element. This may be confusing, since the elements themselves are not necessarily referring to the same object, but it is a useful model to consider them to be visual states of the same conceptual page entity, that we happen to call "element".

4. User-agent keyframes

The following <@keyframes> are added to the HTML user agent style sheet:

@keyframes -ua-view-transition-fade-out {
	to { opacity: 0; }
}

@keyframes -ua-view-transition-fade-in {
	from { opacity: 0; }
}

5. Pseudo-elements

The following view-transition pseudo-elements represent the various items being animated.

The ::view-transition pseudo-element acts as a grouping element for other view-transition pseudo-elements and has the document’s document element as its originating element.

Note: For example, :root::view-transition selector matches this pseudo-element, but div::view-transition does not.

Other view-transition pseudo-elements take a <pt-name-selector> argument to specify which elements named with view-transition-name are affected.

There can be multiple pseudo-elements of the same type, one for each view-transition-name participating in a transition.

The <pt-name-selector> is defined as follows:

<pt-name-selector> = '*' | <custom-ident>

A value of * makes the corresponding selector apply to all pseudo elements of the specified type. The specificity of a view-transition selector with a * argument is zero.

The <custom-ident> value makes the corresponding selector apply to exactly one pseudo element of the specified type, namely the pseudo-element that is created as a result of the view-transition-name property on an element with the same <custom-ident> value. The specificity of a view-transition selector with a <custom-ident> argument is the same as for other pseudo-elements, and is equivalent to a type selector.

The following describes all of the view-transition pseudo-elements and their function:

::view-transition

This pseudo-element is the grouping container of all the other view-transition pseudo-elements. Its originating element is the document’s document element.

The following is added to the HTML user agent style sheet:

html::view-transition {
	position: fixed;
	inset: 0;
}

Note: This pseudo-element provides a containing block for all ::view-transition-group() pseudo-elements. The aim of the style is to size the pseudo-element to cover the snapshot viewport and position all ::view-transition-group() pseudo-elements relative to the snapshot viewport origin.

::view-transition-group( <pt-name-selector> )

One of these pseudo-elements exists for each view-transition-name in a view transition, and holds the rest of the pseudo-elements corresponding to this view-transition-name.

Its originating element is the ::view-transition pseudo-element.

The following is added to the HTML user agent style sheet:

html::view-transition-group(*) {
	position: absolute;
	top: 0;
	left: 0;

	animation-duration: 0.25s;
	animation-fill-mode: both;
}
The aim of the style is to position the element relative to its ::view-transition parent.

In addition to above, styles in the view transition style sheet animate this pseudo-element’s width and height from the size of the old element’s border box to that of the new element’s border box. Also the element’s transform is animated from the old element’s screen space transform to the new element’s screen space transform. This style is generated dynamically since the values of animated properties are determined at the time that the transition begins.

The selector for this and subsequently defined pseudo-elements is likely to change to indicate position in the pseudo-tree hierarchy.

::view-transition-image-pair( <pt-name-selector> )

One of these pseudo-elements exists for each view-transition-name being in a view transition, and holds the images of the old and new elements.

Its originating element is the ::view-transition-group() pseudo-element with the same transition-name.

The following is added to the HTML user agent style sheet:

html::view-transition-image-pair(*) {
	position: absolute;
	inset: 0;

	animation-duration: inherit;
	animation-fill-mode: inherit;
}

Note: The aim of the style is to position the element to occupy the same space as its ::view-transition-group() element.

::view-transition-old( <pt-name-selector> )

One of these pseudo-elements exists for each element in the old DOM being animated by the view transition, and is a replaced element displaying the old element’s snapshot image. It has natural dimensions equal to the snapshot’s size.

Its originating element is the ::view-transition-image-pair() pseudo-element with the same transition-name.

The following is added to the HTML user agent style sheet:

html::view-transition-old(*) {
	position: absolute;
	inset-block-start: 0;
	inline-size: 100%;
	block-size: auto;

	animation-duration: inherit;
	animation-fill-mode: inherit;
}

Note: The aim of the style is to match the element’s inline size while retaining the aspect ratio. It is also placed at the block start.

Note: Additional styles in the view transition style sheet added to animate these pseudo-elements are detailed in setup transition pseudo-elements and update pseudo-element styles.

::view-transition-new( <pt-name-selector> )

Identical to ::view-transition-old(), except it deals with the new element instead.

The precise tree structure, and in particular the order of sibling pseudo-elements, is defined in the setup transition pseudo-elements algorithm.

6. Concepts

6.1. Phases

Phases represent an ordered sequence of states. Since phases are ordered, prose can refer to phases before a particular phase, meaning they appear earlier in the sequence, or after a particular phase, meaning they appear later in the sequence.

The initial phase is the first item in the sequence.

Note: For the most part, a developer using this API does not need to worry about the different phases, since they progress automatically. It is, however, important to understand what steps happen in each of the phases: when the snapshots are captured, when pseudo-element DOM is created, etc. The description of the phases below tries to be as precise as possible, with an intent to provide an unambiguous set of steps for implementors to follow in order to produce a spec-compliant implementation.

6.2. The snapshot viewport

The snapshot viewport covers all areas of the window that could potentially display web content. This area is consistent regardless of root scrollbars or interactive widgets.

"Interactive widgets" refers to UI described within a property definition in CSS Viewport § 3.4 interactive-widget. This should be extracted into an exported definition that can be used independently of the property.

A diagram of a phone screen, including a top status bar, a browser URL bar, web-content area with a floating scrollbar, a virtual keyboard, and a bottom bar with an OS back button The previous diagram, but highlights the area that’s the 'snapshot' viewport, which includes everything except the top status bar and the bottom bar with the OS back button
An example of the snapshot viewport on a mobile OS. The snapshot includes the URL bar, as this can be scrolled away. The keyboard is included as this appears and disappears. The top and bottom bars are part of the OS rather than the browser, so they’re not included in the snapshot viewport.
A diagram of a desktop browser window, including a tab bar, a URL bar, and a web-content area featuring both horizontal and vertical scrollbars The previous diagram, but highlights the area that’s the 'snapshot' viewport, which includes the web content area and the scrollbars
An example of the snapshot viewport on a desktop OS. This includes the scrollbars, but does not include the URL bar, as web content never appears in that area.

This means the snapshot canvas size is likely to be consistent for the document element's old image and new element.

The snapshot viewport origin refers to the start of the block and inline axes of the snapshot viewport.

6.3. The view-transition layer stacking layer

This specification introduces a stacking layer to the CSS 2 §  Elaborate description of Stacking Contexts.

The ::view-transition pseudo-element generates a new stacking context called the view-transition layer with the following characteristics:

Note: The intent of the feature is to be able to capture the contents of the page, which includes the top layer elements. In order to accomplish that, the view-transition layer cannot be a part of the captured top layer context, since that results in a circular dependency. Instead, this stacking context is a sibling of other page contents.

Create a diagram to explain these stacking contexts.

Do we need to clarify that the stacking context for the root and top layer elements has filters and effects coming from the document element's style?

6.4. Captured elements

A captured element is a struct with the following:

old image

an image or null. Initially null.

The type of "image" needs to be linked or defined.

old styles

a set of styles or null. Initially null.

The type of "a set of styles" needs to be linked or defined.

new element

an element or null. Initially null.

The type of "element" needs to be linked or defined.

In addition, a captured element has the following style definitions:

group keyframes

A CSSKeyframesRule or null. Initially null.

group animation name rule

A CSSStyleRule or null. Initially null.

group styles rule

A CSSStyleRule or null. Initially null.

old view-box rule

A CSSStyleRule or null. Initially null.

new view-box rule

A CSSStyleRule or null. Initially null.

image pair isolation rule

A CSSStyleRule or null. Initially null.

view blend mode rule

A CSSStyleRule or null. Initially null.

Note: These are used to update, and later remove styles from a document's view transition style sheet.

6.5. Additions to Document

A Document additionally has:

active DOM transition

a ViewTransition or null. Initially null.

transition suppressing rendering

a boolean. Initially false.

view transition style sheet

a style sheet. Initially a new style sheet in the user-agent origin, ordered after the HTML user agent style sheet.

Note: This is used to hold dynamic styles relating to transitions.

7. API

7.1. Additions to Document

partial interface Document {
  ViewTransition startViewTransition(optional UpdateDOMCallback? callback = null);
};

callback UpdateDOMCallback = Promise<any> ();

7.1.1. startViewTransition()

The method steps for startViewTransition(callback) are as follows:
  1. Let transition be a new ViewTransition object in this’s relevant Realm.

  2. Set transition’s DOM update callback to callback.

  3. Let document be this’s relevant global object’s associated document.

  4. If document’s active DOM transition is not null, then skip the view transition document’s active DOM transition with an "AbortError" DOMException in this’s relevant Realm.

    Note: This can result in two asynchronous DOM update callbacks running concurrently. One for the document’s current active DOM transition, and another for this transition. As per the design of this feature, it’s assumed that the developer is using another feature or framework to correctly schedule these DOM changes.

  5. Set document’s active DOM transition to transition.

    Note: The process continues in setup view transition, via perform pending transition operations, which is called in § 8.1 Monkey patches to rendering.

  6. Return transition.

If the default animations for the view transition are acceptable, then kicking off a transition requires nothing more than setting view-transition-name in the element’s CSS, and a single line of script to start it:
document.startViewTransition(async () => {
	await coolFramework.changeTheDOMToPageB();
});

If more precise management is needed, however, transition elements can be managed in script:

async function doTransition() {
	// Specify "old" elements. The name is used to match against
	// "new" elements they should transition to, and to refer to
	// the transitioning pseudo-element.
	document.querySelector(".old-message").style.viewTransitionName = "message";

	const transition = document.startViewTransition(async () => {
		// This callback is invoked by the browser when "old"
		// capture finishes and the DOM can be switched to the new
		// state. No frames are rendered until this callback returns.

		// DOM changes may be asynchronous
		await coolFramework.changeTheDOMToPageB();

		// Naming elements during the callback marks them as
		// "new", to be matched up with the same-named "old"
		// elements marked previously and transitioned between.
		document.querySelector(".new-message").style.viewTransitionName = "message";

		// viewTransitionNames must be unique, so the above assumes that
		// <code data-opaque bs-autolink-syntax='`.old-message`'>.old-message</code> has been removed/hidden by <code data-opaque bs-autolink-syntax='`changeTheDOMToPageB()`'>changeTheDOMToPageB()</code>.
	});

	// When ready resolves, all pseudo-elements for this transition have
	// been generated.
	// They can now be accessed in script to set up custom animations.
	await transition.ready;

	document.documentElement.animate(keyframes, {
		...animationOptions,
		pseudoElement: "::view-transition-group(message)",
	});

	// When the finished promise resolves, that means the transition is
	// finished.
	await transition.finished;
}

7.2. The ViewTransition interface

[Exposed=Window]
interface ViewTransition {
  undefined skipTransition();
  readonly attribute Promise<undefined> finished;
  readonly attribute Promise<undefined> ready;
  readonly attribute Promise<undefined> domUpdated;
};

Note: The ViewTransition represents and controls a single same-document transition. That is, it controls a transition where the starting and ending document are the same, possibly with changes to the document’s DOM structure.

A ViewTransition has the following:

named elements

a map, whose keys are view transition names and whose values are captured elements. Initially a new map.

phase

One of the following phases:

  1. "pending-capture".

  2. "dom-update-callback-called".

  3. "animating".

  4. "done".

DOM update callback

an UpdateDOMCallback or null. Initially null.

ready promise

a Promise. Initially a new promise in this’s relevant Realm.

DOM updated promise

a Promise. Initially a new promise in this’s relevant Realm.

finished promise

a Promise. Initially a new promise in this’s relevant Realm.

The finished getter steps are to return this’s finished promise.

The ready getter steps are to return this’s ready promise.

The domUpdated getter steps are to return this’s DOM updated promise.

7.2.1. skipTransition()

The method steps for skipTransition() are:
  1. If this's phase is not "done", then skip the view transition for this with an "AbortError" DOMException.

8. Algorithms

8.1. Monkey patches to rendering

Run the following steps before intersection observer steps in the update the rendering steps:
  1. For each fully active Document in docs, perform pending transition operations for that Document.

Note: These steps will be added to the update the rendering in the HTML spec. As such, the prose style is written to match other steps in that algorithm.

  1. For each Document in docs with a transition suppressing rendering of true:

    Move this to the html spec. See html/8368.

    How should input be handled when in this state? The last frame presented to the user will not reflect the DOM state as it asynchronously switches to the new version. See issue 7797.

    Note: The aim is to prevent unintended DOM updates from being presented to the user after a cached snapshot for the elements has been captured. We wait for one rendering opportunity after prepare to present DOM mutations made by the author before prepare to be presented to the user. This is also the content captured in snapshots.

Note: These steps will be added to the update the rendering in the HTML spec. As such, the prose style is written to match other steps in that algorithm.

8.2. Perform pending transition operations

To perform pending transition operations given a Document document, perform the following steps:
  1. If document’s active DOM transition is not null, then:

    1. If document’s active DOM transition's phase is "pending-capture", then setup view transition with document’s active DOM transition.

    2. Otherwise, if document’s active DOM transition's phase is "animating", then update transition DOM for document’s active DOM transition.

8.3. Setup view transition

To setup view transition given a ViewTransition transition, perform the following steps:
  1. Let namedElements be transition’s named elements.

  2. Let usedTransitionNames be a new set of strings.

  3. Let document be transition’s relevant global object’s associated document.

  4. For each element of every Element and pseudo-element connected to document, in paint order:

    The link for "paint order" doesn’t seem right. Is there a more canonical definition?

    1. If any flat tree ancestor of this element skips its contents, then continue.

    2. Let transitionName be the computed value of view-transition-name for element.

    3. If transitionName is none, or element is not rendered, then continue.

    4. If any of the following is true:

      Then skip the view transition for transition with an "InvalidStateError" DOMException in transition’s relevant Realm, and return.

    5. Append transitionName to usedTransitionNames.

    6. Let capture be a new captured element struct.

    7. Set capture’s old image to the result of capturing the image of element.

    8. Set capture’s old styles to the following:

      transform

      A CSS transform that would place element from the snapshot viewport origin to its current quad.

      This value is identity for the document element.

      width
      height

      The size of the snapshot viewport if element is the document element, otherwise, the width and height of element’s border box.

      object-view-box

      none if element is the document element, otherwise, an object-view-box value that, when applied to the old image, will cause the view box to coincide with element’s border box in the image. This must be expressed using inset().

      writing-mode

      The writing-mode of element.

      direction

      The direction of element.

      This needs proper types.

    9. Set namedElements[transitionName] to capture.

  5. Set document’s transition suppressing rendering to true.

  6. Queue a global task on the DOM manipulation task source, given transition’s relevant global object, to execute the following steps:

    Note: A task is queued here because the texture read back in capturing the image may be async, although the render steps in the HTML spec act as if it’s synchronous.

    "DOM manipulation task source" doesn’t link due to a bikeshed bug.

    1. If transition’s phase is "done", then abort these steps.

      Note: This happens if transition was skipped before this point.

    2. Call the DOM update callback of transition.

    3. React to transition’s DOM updated promise:

8.4. Skip the view transition

To skip the view transition for ViewTransition transition with reason reason:
  1. Let document be transition’s relevant global object’s associated document.

  2. Assert: document’s active DOM transition is transition.

  3. Assert: transition’s phase is not "done".

  4. If transition’s phase is before "dom-update-callback-called", then call the DOM update callback of transition.

  5. Set transition suppressing rendering to false.

  6. If transition’s phase is equal to or after "animating", then clear view transition transition.

  7. Set transition’s phase to "done".

  8. Set document’s active DOM transition to null.

  9. If transition’s ready promise has not yet been resolved, reject it with reason.

    Note: The ready promise would’ve been resolved if skipTransition() is called after we start animating.

  10. React to transition’s DOM updated promise:

8.5. Capture the image

To capture the image given an Element element, perform the following steps. They return an image.
  1. If element is the document element, then:

    1. Render the region of the element and the top layer that intersects the snapshot viewport, on a transparent canvas the size of the snapshot viewport, following the capture rendering characteristics, and these additional characteristics:

      • Areas outside element’s scrolling box should be rendered as if they were scrolled to, without moving or resizing the layout viewport. This must not trigger events related to scrolling or resizing, such as IntersectionObservers.

        "scrolling box" and "layout viewport" are not exported. They either need exporting, or replaced with better references.

        A phone browser window, showing a URL bar, a fixed-position element directly beneath it, and some page content beneath that. A scroll bar indicates the page has been scrolled significantly. The captured snapshot. It shows that content beneath the URL bar was included in the capture.
        An example of what the user sees compared to the captured snapshot. This example assumes the root is the only element with a transition name.
      • Areas that cannot be scrolled to (i.e. they are out of scrolling bounds), should render the canvas background.

        A phone browser window, showing a URL bar, and some content beneath. A scroll bar indicates the page is scrolled to the top. The captured snapshot. It shows the area underneath the URL bar as the same color as the rest of the document.
        An example of what the user sees compared to the captured snapshot. This example assumes the root is the only element with a transition name.
    2. Return the canvas as an image. The natural size of the image is equal to the snapshot viewport.

  2. Otherwise:

    1. Render element and its descendants, at the same size it appears in its node document, over an infinite transparent canvas, following the capture rendering characteristics.

    2. Let interestRectangle be the result of computing the interest rectangle for element.

      Note: The interestRectangle is the subset of element’s ink overflow rectangle that should be captured. This is required for cases where an element’s ink overflow rectangle needs to be clipped because of hardware constraints. For example, if it exceeds the maximum texture size.

    3. Return the portion of the canvas within interestRectangle as an image. The natural size of the image is equal to the interestRectangle bounds.

The capture rendering characteristics are as follows:

8.6. Update transition DOM

To update transition DOM given a ViewTransition transition:
  1. Let document be transition’s relevant global object’s associated document.

  2. Let hasActiveAnimations be a boolean, initially false.

  3. For each view-transition pseudo-elements associated with transition:

    1. Let element be the view-transition pseudo-element.

    2. For each animation whose timeline is a document timeline associated with document, and contains at least one associated effect whose effect target is element, set hasActiveAnimations to true if any of the following conditions is true:

      The prose around "effect target" is incorrect, but #8001 needs to land before it can be fixed.

  4. If hasActiveAnimations is false:

    1. Set transition’s phase to "done".

    2. Clear view transition transition.

    3. Set document’s active DOM transition to null.

    4. Resolve transition’s finished promise.

    5. Return.

  5. Update pseudo-element styles for transition.

    Note: The above implies that a change in incoming element’s size or position will cause a new keyframe to be generated. This can cause a visual jump. We could retarget smoothly but don’t have a use-case to justify the complexity. See issue 7813 for details.

8.7. Compute the interest rectangle

To compute the interest rectangle of an Element element, perform the following steps. They return a rectangle.
  1. Assert: element is not element’s node document's document element.

    Note: The document element is captured differently, as specified in capture the image.

  2. If element’s ink overflow area does not exceed an implementation-defined maximum size, then return a rectangle that is equal to element’s ink overflow rectangle.

  3. Otherwise:

    Define the algorithm used to clip the snapshot when it exceeds max size.

8.8. Setup transition pseudo-elements

To setup transition pseudo-elements for a ViewTransition transition:
  1. Let document be this’s relevant global object’s associated document.

  2. Let transitionRoot be the result of creating a new ::view-transition pseudo-element in document’s snapshot viewport.

  3. For each transitionNamecapturedElement of transition’s named elements:

    1. Let group be the result of creating a new ::view-transition-group() pseudo-element with the name transitionName.

      "name" should be defined/linked.

    2. Append group to transitionRoot.

      This should be better defined. I’m not sure if pseudo-elements have defined ways to modify their DOM.

    3. Let imagePair be a new ::view-transition-image-pair() pseudo-element with the name transitionName.

    4. Append imagePair to group.

    5. If capturedElement’s old image is not null, then:

      1. Let old be a new ::view-transition-old() replaced element pseudo-element, with the name transitionName, displaying capturedElement’s old image.

      2. Append old to imagePair.

      3. Let oldViewBox be capturedElement’s old styles object-view-box property.

      4. Set capturedElement’s old view-box rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet.

        html::view-transition-old(transitionName) {
          object-view-box: oldViewBox;
        }
        

        Note: The above code example contains variables to be replaced.

    6. If capturedElement’s new element is not null, then:

      1. Let new be a new ::view-transition-new() replaced element pseudo-element, with the name transitionName, displaying the capture the image of capturedElement’s new element.

      2. Append new to imagePair.

      The new element and its contents (the flat tree descendants of the element, including both text and elements, or the replaced content of a replaced element), except the view-transition pseudo-elements, are not painted (as if they had visibility: hidden) and do not respond to hit-testing (as if they had pointer-events: none) until new exists.

    7. If both of capturedElement’s old image and new element are not null, then:

      1. Let transform be capturedElement’s old styles's transform property.

      2. Let width be capturedElement’s old styles's width property.

      3. Let height be capturedElement’s old styles's height property.

      4. Set capturedElement’s group keyframes to a new CSSKeyframesRule representing the following CSS, and append it to document’s view transition style sheet:

        @keyframes -ua-view-transition-group-anim-transitionName {
          from {
            transform: transform;
            width: width;
            height: height;
          }
        }
        

        Note: The above code example contains variables to be replaced.

      5. Set capturedElement’s group animation name rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet:

        html::view-transition-group(transitionName) {
          animation-name: -ua-view-transition-group-anim-transitionName;
        }
        

        Note: The above code example contains variables to be replaced.

      6. Set capturedElement’s image pair isolation rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet:

        html::view-transition-image-pair(transitionName) {
          isolation: isolate;
        }
        

        Note: The above code example contains variables to be replaced.

      7. Set capturedElement’s view blend mode rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet:

        html::view-transition-old(transitionName),
        html::view-transition-new(transitionName) {
          mix-blend-mode: plus-lighter;
        }
        

        Note: The above code example contains variables to be replaced.

        Note: mix-blend-mode: plus-lighter ensures that the blending of identical pixels from the old and new images results in the same color value as those pixels, and achieves a "correct" cross-fade.

        Isolation and the dynamic setting of blending is only necessary to get the right cross-fade between new and old image pixels. Would it be simpler to always add it and try to optimize in the implementation?

8.9. Update pseudo-element styles

To update pseudo-element styles for a ViewTransition transition:
  1. For each transitionNamecapturedElement of transition’s named elements:

    1. Let width, height, transform, writingMode, and direction be null.

    2. If capturedElement’s new element is null, then:

      1. Set width to capturedElement’s old styles width property.

      2. Set height to capturedElement’s old styles height property.

      3. Set transform to capturedElement’s old styles transform property.

      4. Set writingMode to capturedElement’s old styles writing-mode property.

      5. Set direction to capturedElement’s old styles direction property.

    3. Otherwise:

      1. Set width to the current width of capturedElement’s new element's border box.

      2. Set height to the current height of capturedElement’s new element's border box.

      3. Set transform to a transform that maps the capturedElement’s new element's border box from document origin to its quad from the snapshot viewport origin.

      4. Set writingMode to the computed value of writing-mode on capturedElement’s new element.

      5. Set direction to the computed value of direction on capturedElement’s new element.

    4. If capturedElement’s group styles rule is null, then set capturedElement’s group styles rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet.

      Otherwise, update capturedElement’s group styles rule to match the following CSS:

      html::view-transition-group(transitionName) {
        width: width;
        height: height;
        transform: transform;
        writing-mode: writingMode;
        direction: direction;
      }
      

      Note: The above code example contains variables to be replaced.

    5. If capturedElement’s new element is not null, then:

      1. Let new be the ::view-transition-new() replaced element pseudo-element, with the name transitionName, displaying the capture the image of capturedElement’s new element.

      2. Let newViewBox be an object-view-box value that when applied to new, will cause the view box to coincide with capturedElement’s new element's border box in the image. This must be expressed using inset().

      3. If capturedElement’s new view-box rule is null, then set capturedElement’s new view-box rule to a new CSSStyleRule representing the following CSS, and append it to document’s view transition style sheet.

        Otherwise, update capturedElement’s new view-box rule to match the following CSS:

        html::view-transition-new(transitionName) {
          object-view-box: newViewBox;
        }
        

        Note: The above code example contains variables to be replaced.

This algorithm must be executed to update styles in user-agent origin if its effects can be observed by a web API.

Note: An example of such a web API is window.getComputedStyle(document.documentElement, "::view-transition").

To call the DOM update callback of a ViewTransition transition:
  1. Assert: transition’s phase is before "dom-update-callback-called".

  2. Let callbackPromise be a new promise in transition’s relevant Realm.

  3. Set transition’s phase to "dom-update-callback-called".

  4. React to callbackPromise:

8.10. Clear view transition

To clear view transition of a ViewTransition transition:
  1. Let document be transition’s relevant global object’s associated document.

  2. Remove all associated view-transition pseudo-elements from document.

    There needs to be a definition/link for "remove".

    There needs to be a definition/link for "associated".

  3. For each capturedElement of transition’s named elements' values:

    1. For each style of capturedElement’s style definitions:

      1. If style is not null, then remove style from document’s view transition style sheet.

Privacy Considerations

This specification introduces no new privacy considerations.

Security Considerations

The images generated using capture the image algorithm could contain cross-origin data (if the Document is embedding cross-origin resources) or sensitive information like visited links. The implementations must ensure this data can not be accessed by the Document. This should be feasible since access to this data should already be prevented in the default rendering of the Document.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[COMPOSITING-1]
Rik Cabanier; Nikos Andronikos. Compositing and Blending Level 1. 13 January 2015. CR. URL: https://www.w3.org/TR/compositing-1/
[CSS-2022]
Tab Atkins Jr.; Elika Etemad; Florian Rivoal. CSS Snapshot 2022. 22 November 2022. NOTE. URL: https://www.w3.org/TR/css-2022/
[CSS-ANIMATIONS-1]
Dean Jackson; et al. CSS Animations Level 1. 11 October 2018. WD. URL: https://www.w3.org/TR/css-animations-1/
[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. 3 November 2022. WD. URL: https://www.w3.org/TR/css-box-4/
[CSS-BREAK-4]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 4. 18 December 2018. WD. URL: https://www.w3.org/TR/css-break-4/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 13 January 2022. CR. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley; Lea Verou. CSS Color Module Level 4. 1 November 2022. CR. URL: https://www.w3.org/TR/css-color-4/
[CSS-CONTAIN-1]
Tab Atkins Jr.; Florian Rivoal. CSS Containment Module Level 1. 25 October 2022. REC. URL: https://www.w3.org/TR/css-contain-1/
[CSS-CONTAIN-2]
Tab Atkins Jr.; Florian Rivoal; Vladimir Levin. CSS Containment Module Level 2. 17 September 2022. WD. URL: https://www.w3.org/TR/css-contain-2/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 18 November 2022. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. 17 December 2020. CR. URL: https://www.w3.org/TR/css-images-3/
[CSS-IMAGES-4]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Image Values and Replaced Content Module Level 4. 13 April 2017. WD. URL: https://www.w3.org/TR/css-images-4/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 23 December 2021. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-SCOPING-1]
Tab Atkins Jr.; Elika Etemad. CSS Scoping Module Level 1. 3 April 2014. WD. URL: https://www.w3.org/TR/css-scoping-1/
[CSS-SHAPES-1]
Rossen Atanassov; Alan Stearns. CSS Shapes Module Level 1. 15 November 2022. CR. URL: https://www.w3.org/TR/css-shapes-1/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 3. 17 December 2021. WD. URL: https://www.w3.org/TR/css-sizing-3/
[CSS-TRANSFORMS-1]
Simon Fraser; et al. CSS Transforms Module Level 1. 14 February 2019. CR. URL: https://www.w3.org/TR/css-transforms-1/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 19 October 2022. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-VIEWPORT-1]
CSS Viewport Module Level 1 URL: https://drafts.csswg.org/css-viewport-1/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 10 December 2019. REC. URL: https://www.w3.org/TR/css-writing-modes-3/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSSOM-1]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). 26 August 2021. WD. URL: https://www.w3.org/TR/cssom-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[FILTER-EFFECTS-1]
Dirk Schulze; Dean Jackson. Filter Effects Module Level 1. 18 December 2018. WD. URL: https://www.w3.org/TR/filter-effects-1/
[FULLSCREEN]
Philip Jägenstedt. Fullscreen API Standard. Living Standard. URL: https://fullscreen.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[INTERSECTION-OBSERVER]
Stefan Zager; Emilio Cobos Álvarez. Intersection Observer. 6 July 2022. WD. URL: https://www.w3.org/TR/intersection-observer/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[SELECTORS-3]
Tantek Çelik; et al. Selectors Level 3. 6 November 2018. REC. URL: https://www.w3.org/TR/selectors-3/
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 11 November 2022. WD. URL: https://www.w3.org/TR/selectors-4/
[WEB-ANIMATIONS-1]
Brian Birtles; et al. Web Animations. 8 September 2022. WD. URL: https://www.w3.org/TR/web-animations-1/
[WEBIDL]
Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
view-transition-name none | <custom-ident> none all elements no n/a discrete per grammar as specified

IDL Index

partial interface Document {
  ViewTransition startViewTransition(optional UpdateDOMCallback? callback = null);
};

callback UpdateDOMCallback = Promise<any> ();

[Exposed=Window]
interface ViewTransition {
  undefined skipTransition();
  readonly attribute Promise<undefined> finished;
  readonly attribute Promise<undefined> ready;
  readonly attribute Promise<undefined> domUpdated;
};

Issues Index

This sections can benefit from diagrams.
The selector for this and subsequently defined pseudo-elements is likely to change to indicate position in the pseudo-tree hierarchy.
"Interactive widgets" refers to UI described within a property definition in CSS Viewport § 3.4 interactive-widget. This should be extracted into an exported definition that can be used independently of the property.
Create a diagram to explain these stacking contexts.
Do we need to clarify that the stacking context for the root and top layer elements has filters and effects coming from the document element's style?
The type of "image" needs to be linked or defined.
The type of "a set of styles" needs to be linked or defined.
The type of "element" needs to be linked or defined.
Move this to the html spec. See html/8368.
How should input be handled when in this state? The last frame presented to the user will not reflect the DOM state as it asynchronously switches to the new version. See issue 7797.
The link for "paint order" doesn’t seem right. Is there a more canonical definition?
This needs proper types.
"DOM manipulation task source" doesn’t link due to a bikeshed bug.
The link for "paint order" doesn’t seem right. Is there a more canonical definition?
"scrolling box" and "layout viewport" are not exported. They either need exporting, or replaced with better references.
Refactor this so the algorithm takes a set of elements that will be captured. This centralizes the logic for deciding if an element should be included or not.
Specify what happens with mix-blend-mode.
The prose around "effect target" is incorrect, but #8001 needs to land before it can be fixed.
This prose isn’t quite right, but it’s blocked on #8004.
Define the algorithm used to clip the snapshot when it exceeds max size.
"name" should be defined/linked.
This should be better defined. I’m not sure if pseudo-elements have defined ways to modify their DOM.
Isolation and the dynamic setting of blending is only necessary to get the right cross-fade between new and old image pixels. Would it be simpler to always add it and try to optimize in the implementation?
There needs to be a definition/link for "remove".
There needs to be a definition/link for "associated".