This CSS module describes the contain property, which indicates that the element’s subtree is independent of the rest of the page. This enables heavy optimizations by user agents when used well.
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 is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
Please send feedback
by filing issues in GitHub (preferred),
including the spec code “css-contain” in the title, like this:
“[css-contain] …summary of comment…”.
All issues and comments are archived.
Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.
Efficiently rendering a website relies on the user agent being able to detect what parts of the page are being displayed,
which parts might affect the currently-displayed section,
and what can be ignored.
There are various heuristics that can be used to guess when a given sub-tree is independent of the rest of the page in some manner,
but they’re fragile,
so innocuous changes to a page may inadvertently make it fail such heuristic tests,
causing rendering to fall into a slow code path.
There are also many things that would be good to isolate which are difficult or impossible to detect in a heuristic manner.
To alleviate these problems
and allow strong, predictable isolation of a subtree from the rest of the page,
this specification defines a contain property.
To allow even further optimization of off-screen contents,
this spec also defines a content-visibility property,
enabling the user agent to skip an element’s layout and painting entirely when not needed.
1.1. Module Interactions
This document defines new features not present in earlier specifications.
In addition, it aims to replace and supersede [CSS-CONTAIN-1] once stable.
1.2. Value Definitions
This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3].
Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords as their property value.
For readability they have not been repeated explicitly.
User agents are expected to support this property on all media, including non-visual ones.
The contain property allows an author to indicate that an element and its contents are,
as much as possible, independent of the rest of the document tree.
This allows user agents to utilize much stronger optimizations when rendering a page using contain properly,
and allows authors to be confident that their page won’t accidentally fall into a slow code path
due to an innocuous change.
none
This value indicates that the property has no effect.
The element renders as normal,
with no containment effects applied.
strict
This value computes to size layout paint style,
and thus turns on all forms of containment for the element.
Tests
Note:contain: content is reasonably "safe" to apply widely;
its effects are fairly minor in practice,
and most content won’t run afoul of its restrictions.
However, because it doesn’t apply size containment,
the element can still respond to the size of its contents,
which can cause layout-invalidation to percolate further up the tree than desired.
Use contain: strict when possible,
to gain as much containment as you can.
size
The value turns on size containment for the element.
This ensures that the containment box can be laid out
without needing to examine its descendants.
Tests
This value turns on layout containment for the element.
This ensures that the containment box is totally opaque for layout purposes;
nothing outside can affect its internal layout,
and vice versa.
Tests
This value turns on style containment for the element.
This ensures that,
for properties which can have effects on more than just an element and its descendants,
those effects don’t escape the element.
Tests
This value turns on paint containment for the element.
This ensures that the descendants of the containment box don’t display outside its bounds,
so if an element is off-screen or otherwise not visible,
its descendants are also guaranteed to be not visible.
Tests
contain is useful when used widely on a page,
particularly when a page contains a lot of "widgets" which are all independent.
For example, assume a micropost social network had markup something like this:
<body><aside>...</aside><section><h2>Messages</h2><article>
Lol, check out this dog: images.example.com/jsK3jkl
</article><article>
I had a ham sandwich today. #goodtimes
</article><article>
I have political opinions that you need to hear!
</article>
…
</section></body>
There are probably a lot of messages displayed on the site,
but each is independent and won’t affect anything else on the site.
As such, each can be marked with contain: content to communicate this to the user agent,
so it can optimize the page and skip a lot of computation for messages that are off-screen.
If the size of each message is known ahead of time, contain: strict can be applied to communicate further restrictions.
Note: Several properties beyond contain can turn on various containments for an element.
These do not affect the value of contain;
an element can have contain: none but still have layout containment turned on by content-visibility,
for example.
3. Types of Containment
There are several varieties of containment that an element can be subject to,
restricting the effects that its descendants can have on the rest of the page in various ways. Containment enables much more powerful optimizations by user agents,
and helps authors compose their page out of functional units,
as it limits how widely a given change can affect a document.
Specification authors introducing new properties or mechanisms
need to consider whether and how the various types of containment
affect what they are introducing,
and include in their specification any effect not described here.
3.1. Size Containment
Giving an element size containment makes its principal box a size containment box and has the following effects:
Note: This affects explicit invocations of the min-content or max-content keywords,
as well as any calculation that depends on these measurement,
such as sizing grid tracks into which a size contained item is placed,
or if fit-content sizing the containment box’s parent.
Laying out a size containment box and its content is conceptually done in two phases:
Sizing as if empty
The usedwidth and height of the containment box are determined as if performing a normal layout of the box,
except that it is treated as having no content—not even through pseudo elements such as ::before, ::after, or ::marker.
Tests
All CSS properties of the size containment box are taken into account
as they would be when performing layout normally.
Other specifications may make specific exemptions.
Note: Even when the element’s sizing properties specify an intrinsic size,
this does not necessarily make the element zero-sized:
properties set on the element itself
continue to be taken into account,
which can cause it to be larger.
Note: Internal table boxes,
which do not include table captions,
are excluded,
because the table layout algorithm
does not allow boxes to become smaller than their inflow content.
Sizing a table cell as if it was empty and then laying out its content inside without changing the size
is effectively an undefined operation.
Manually setting the width or height properties to 0 cannot make it smaller than its content.
This concern does not apply to table captions,
which are perfectly capable of having a fixed size
that is independent of their content.
By itself, size containment does not offer much optimization opportunity.
Its primary benefit on its own is that tools which want to lay out the containment box's contents
based on the containment box's size
(such as a JS library implementing the "container query" concept)
can do so without fear of "infinite loops",
where having a child’s size respond to the size of the containment box causes the containment box's size to change as well,
possibly triggering further changes in how the child sizes itself
and possibly thus more changes to the containment box's size,
ad infinitum.
When paired with layout containment, though,
possible optimizations that can be enabled include (but are not limited to):
When the style or contents of a descendant of the containment box is changed,
calculating what part of the DOM tree is "dirtied" and might need to be re-laid out
can stop at the containment box.
When laying out the page,
if the containment box is off-screen or obscured,
the layout of its contents (i.e. "laying out in-place") can be delayed or done at a lower priority.
3.2. Inline-Size Containment
Giving an element inline-size containment applies size containment to the inline-axis sizing of its principal box.
This means the inline-axisintrinsic sizes of the principal box are determined as if the element had no content.
However, content continues to impact the box’s block-axisintrinsic sizes as usual,
and the box is allowed to fragment normally in the block axis.
Note: In some cases,
a box’s block-axisintrinsic sizes can impact layout in the parent formatting context in ways that affect the box’s inline size (e.g. by triggering scrollbars on an ancestor element),
creating a dependency of the box’s inline size on its own content.
If this changed inline size results in a different block size,
that new block size can loop into further impacting the parent formatting context,
but not in a way that reverts it to the previously-problematic layout.
For example, if scrollbars were introduced,
they are not then removed,
even if the consequent block size is small enough to not need them;
or if a box’s logical height collides with a lower-placed float and is cleared down
to where it also has more available inline space
and thus becomes short enough to not have collided,
it is not them moved back up to its previous problematic size and position.
In general, the relationship between an element’s inline size
and it’s block size
is unpredictable and non-monotonic,
with the block size capable of shifting up and down arbitrarily
as the inline size is changed.
Infinite cycles are prevented
by ensuring that layout does not revert to a previous (known-problematic) state,
even if a naive analysis of the constraints would allow for such;
in other words, layout always “moves forward”.
We believe that current CSS layout specifications incorporate such rules,
but to the extent that they don’t,
please inform the CSSWG so that these errors can be corrected.
Consider this example,
where float placement creates a dependency of block sizes on inline sizes:
The block layout algorithm will first place the floating boxes,
with the first two sitting in the left and right corners of the container,
and the third, being too wide to fit between, being pushed below them.
The following article will then be laid out.
Because it is display: flow-root,
it cannot intersect any floats,
and thus must take them into account
when figuring out how to size and position itself.
The layout engine first attempts to place the article flush with the top of the container,
resulting a 100px width,
plenty wide enough to accommodate its min-content size.
However, due to the aspect-ratio of its child,
this would cause the article to be 100px tall as well,
which would intersect the third float 80px below,
so this layout opportunity is discarded.
It then attempts to position the article flush with the top of the third float,
in the narrow 40px-wide space to its right.
However, since the article’s min-width makes it too large
to fit in the 40px-wide space beside the third float,
it shifts below that one as well,
forming a 200px square below all the floated boxes.
Article
If the min-width is removed from the article,
or if inline-size containment is added to
either the article or header (causing min-width: min-content to resolve to zero),
then the article will fit as a 40px square
next to the final floated div (possibly with some of its content overflowing).
At this point, the width and height of the article (40px each) would fit back in the first considered space,
flush with the top of the container.
However, the box is not returned to the previous position,
because the layout engine knows already
that this position would result in an invalid layout.
If at least one fragmentation container of a fragmentation context has layout containment,
or if at least one fragmentation container of a fragmentation context is a descendant of layout containment boxand at least one subsequent fragmentation container of the same fragmentation context is not a descendant of that same element with layout containment,
then the first layout containment box which is either a fragmentation container itself
or is an ancestor of a fragmentation container must “trap” the remainder of the fragmented flow: fragmentation must not continue past the layout containment boundary,
and the last fragmentation container within the first layout containment boundary
is treated as if it is the last fragmentation container in its fragmentation context.
If subsequent fragmentation containers in the fragmentation context are only generated when more content remains in the fragmented flow,
then they are not generated.
If they would exist regardless,
they remain part of the fragmentation context,
but do not receive any content from the fragmented flow.
Note: At the time of writing, no stable specification is affected by this point.
Only specifications that would enable some (but not all) fragmentation containers of a fragmentation context
to be layout-contained (or descendants of a layout contained element)
are concerned.
This is not the case of [CSS-PAGE-3] nor of [CSS-MULTICOL-1].
This requirement is nonetheless included because
several mechanisms that would make this a possibility have been considered
(e.g.: [CSS-REGIONS-1], ::nth-fragment(), a hypothetical selector for individual columns of a multicol…),
and the guarantees that layout containment is intended to offer would not be realized
if such mechanisms did not abide by this rule. [CSS-REGIONS-1] has details over how layout containment affects
regions.
In this [CSS-REGIONS-1] example,
content can flow from #a to #b,
from #b to #c.
However as #c is the last fragment container in the first layout containment box it traps all the remaining content,
and nothing gets flowed into #d, #e, or #f.
If the computed value of the overflow property is
either visible or clip or a combination thereof,
any overflow must be treated as ink overflow.
For the purpose of the vertical-align property,
or any other property whose effects
need to relate the position of the layout containment box's baseline
to something other than its descendants,
the containment box is treated as having no baseline.
Possible optimizations that can be enabled by layout containment include (but are not limited to):
When laying out the page,
the contents of separate containment boxes can be laid out in parallel,
as they’re guaranteed not to affect each other.
When laying out the page,
if the containment box is off-screen or obscured
and the layout of the visible parts of the screen do not depend on the size of the containment box (for example, if the containment box is near the end of a block container,
and you’re viewing the beginning of the block container),
the layout of the containment box' contents can be delayed or done at a lower priority.
(When paired with size containment,
this optimization can be applied more liberally.)
3.4. Style Containment
Giving an element style containment has the following effects:
Note: This implies that the depth of quote nesting in the subtree
is unchanged and starts at the value that its context normally implies,
but that changes to the depth of quote nesting by these values inside the subtree
do not affect the depth of quote nesting outside the subtree.
A scoped property has its effects scoped to a particular element or subtree.
If scoped to an element,
it must act as if the scoping element was the root of the document
for the purpose of evaluating the property’s effects:
any uses of the property outside the scoping element must have no effect on the uses of the property on or in the scoping element,
and vice versa.
Note: “Scoping to an element” is currently unused.
It is defined as an extension point for future specifications to use.
If scoped to a sub-tree, it’s the same,
except the scoping element itself is counted as "outside" the tree,
like the rest of the document,
and the effects of the property on that element are unaffected by scoping.
When considering the effects of the scoped property on elements inside the subtree,
the element at the base of the subtree is treated as if it was the root of the document.
As counter-increment is scoped to an element’s subtree,
the first use of it within the subtree
creates a fresh counter,
regardless of whether the counter had been used outside the scoping element.
Any increments made within the subtree have no effect on counters of the same name outside the scoping element.
However, the counter() and counters() value of the content property is not itself scoped,
and can refer to counters established outside of the subtree.
body {counter-reset: foo;}
div {counter-increment: foo;/* and to help observe nesting properly */
margin: .2em;padding:.2em;background: #0001;}
div::before {content:counters(foo,".");}
Will render like:
1
2
2.1
2.2
3
4
5
Here, the style-contained element still interacts with the foo counter normally,
incrementing it to 2.
Its ::before pseudo-element,
which is effectively a child and thus within the containment boundary,
can still see that counter normally,
and thus will render the string "2".
Later siblings of the style-contained element don’t see anything that goes on within the boundary,
so they increment the foo counter to 3 and beyond.
Within the containment, however,
the descendant divs aren’t allowed to modify the value of the outside counter,
so instead they’ll create a fresh (nested) foo counter
when they attempt to increment it.
3.4.1. Possible Style-Containment Optimizations
This section is non-normative.
Possible optimizations that can be enabled by style containment include (but are not limited to):
Whenever a property is changed on a descendant of an element with style containment,
calculating what part of the DOM tree is "dirtied" and might need to have its style recalculated
can stop at the element with style containment.
3.5. Paint Containment
Giving an element paint containment makes its principal box a paint containment box and has the following effects:
The contents of the element
including any ink or scrollable overflow must be clipped to the overflow clip edge of the paint containment box,
taking [[css-backgrounds-3#corner clipping|corner clipping]] into account.
This does not include the creation of any mechanism
to access or indicate the presence of the clipped content;
nor does it inhibit the creation of any such mechanism through other properties, such as overflow, resize, or text-overflow.
Note: The behavior is described in this paragraph is equivalent to changing overflow-x: visible into overflow-x: clip and overflow-y: visible into overflow-y: clip at used value time,
while leaving other values of overflow-x and overflow-y unchanged.
Possible optimizations that can be enabled by paint containment include (but are not limited to):
If the containment box is off-screen or obscured,
the UA can usually skip trying to paint its contents,
as they’re guaranteed to be off-screen/obscured as well.
Note: Some paint effects such as the blur() filter from [FILTER-EFFECTS-1] have non local effects.
The user agent needs to keep track of these,
as it may need to repaint parts of an element with such a filter
when its descendents change,
even if they have paint containment and could otherwise be skipped.
Unless the clipped content is made accessible via a separate mechanism
such as the overflow, resize, or text-overflow properties,
the UA can reserve "canvas" space for the box exactly the box’s size.
(In similar, scrollable, situations, like overflow: hidden,
it’s possible to scroll to the currently-clipped content,
so UAs often predictively overpaint somewhat
so there’s something to see as soon as the scroll happens,
rather than a frame later.)
Because they are guaranteed to be stacking contexts,
scrolling elements can be painted into a single GPU layer.
4. Suppressing An Element’s Contents Entirely: the content-visibility property
Tests
Tests related to content-visibility which haven't been triaged to any particular point in the spec
these should be more accurately dispatched paragraph per paragraph to the relevant points in the specification
The content-visibility property
controls whether or not an element renders its contents at all,
along with forcing a strong set of containments,
allowing user agents to potentially omit large swathes of layout and rendering work
until it becomes needed.
It has the following values:
visible
No effect.
The element’s contents are laid out and rendered as normal.
Unlike hidden,
the skipped contentsmust still be available as normal
to user-agent features such as find-in-page, tab order navigation, etc.,
and must be focusable and selectable as normal.
The user agent should additionally avoid as much layout/rendering work as possible
for skipped contents;
the combination of heavy containment and making the contents invisible and untouchable
enables heavy optimizations.
If rendering work is done at some point,
the user-agent should retain the previously computed layout state if possible,
to allow the skipped contents to be displayed quickly at a later moment.
layout containment ensures that
the user-agent is able to omit layout work in skipped subtrees,
since the results of such layouts will not affect elements outside of the container element.
style containment ensures that
counters do not have to be processed in skipped subtrees,
since they do not affect counters outside of the container element.
paint containment ensures that
ink overflow of painted contents is clipped;
this, in turn, means that user-agent can reliably determine
when the visible portion of the element approaches the viewport
(and, for content-visibility: auto, start painting it).
size containment ensures that
the user-agent is able to omit layout in skipped subtrees,
since the results of such layouts
will not affect the container element’s size.
An element that has content-visibility: auto is in one of three states
when it comes to its proximity to the viewport:
The element is close to the viewport:
In this state, the element is considered "on-screen":
its paint containment box's overflow clip edge intersects with the viewport,
or a user-agent defined margin around the viewport.
Note: This margin is meant to allow the user agent
to begin preparing for an element to be in the viewport soon.
A margin of 50% is suggested as a reasonable default.
If a filter (see [FILTER-EFFECTS-1]) with non local effects includes the element as part of its input,
the user agent should also treat the element as relevant to the user
when the filter’s output can affect the rendering within the viewport
(or within the user-agent defined margin around the viewport),
even if the element itself is still off-screen.
The element is far away from the viewport:
In this state, the element’s proximity to the viewport has been computed and is not close to the viewport.
The element’s proximity to the viewport is not determined:
In this state, the computation to determine the element’s proximity to the viewport has not been done
since the last time the element was connected.
Initially, the element’s proximity to the viewport is not determined.
Only connected elements with content-visibility: auto determine their proximity to the viewport.
When the element becomes disconnected, the element’s proximity to the viewport becomes not determined.
An element is relevant to the user if any of the following conditions are true:
content-visibility: hidden lays powerful restrictions onto an element,
and so should be used with caution.
It also enables some very useful scenarios,
often improving on existing techniques,
a few of which are outlined here.
If a page needs to take some measurements of elements or text
which aren’t themselves going to be rendered,
commonly this is done by positioning the stuff-to-be-measured off-screen,
using something like position: absolute; left: -100000px;,
then calling an API like getBoundingClientRect().
Unfortunately, even though the page never intends to display this content,
the user agent will still have to do full styling, layout, and rendering for the content, just in case it affects what’s shown on screen.
The author also can’t,
without further work,
guarantee that the content won’t accidentally show up on-screen;
even a very negative left value (like above) might not be enough,
depending on the content.
Wrapping this content in a content-visibility: hidden container
solves all of these problems.
If the wrapper has no border, background, etc,
then it and its skipped contents are guaranteed to never render anything to the screen,
no matter how big they get.
Because the contents are skipped,
the user agent can also avoid styling or laying them out
until absolutely necessary,
when script finally asks for it.
A "single-page app" often consists of several independent panes or "views",
of which only one is displayed at a time.
If the author wants to avoid paying styling/layout/rendering/etc cost
for the inactive views,
they can remove them from the document entirely,
or at minimum apply display:none to them.
Unfortunately,
this means that when the view does need to be displayed,
all of the styling/layout/rendering/etc work needs to be done all at once,
potentially causing a noticeable delay before the view actually shows up.
Alternately, the view can just be positioned off-screen.
This means it’ll be immediately ready when it’s time to be used,
but it incurs the cost of styling/layout/rendering all the time,
which might be significant,
especially if there are a number of inactive views.
The inactive views also might still show up to accessibility tooling,
confusing users of screen-readers,
people using Ctrl-F to find-in-page, etc.
content-visibility: hidden improves on both of these options.
Because the contents are skipped,
the user agent isn’t spending time on them when they’re not active.
They’re also not visible to screen readers,
find-in-page,
and other tools.
And because user agents should preserve previous styling/layout work if possible,
if the view was displayed before,
re-rendering it might be very fast.
If an author wants to make an element "invisible",
but still show up in the page for layout purposes,
one option is visibility: hidden.
However, descendants of a visibility: hidden element
can set visibility: visible and start showing up again,
which isn’t always intuitive or expected.
content-visibility: hidden performs a very similar purpose,
but descendants can’t turn it "off" and start displaying;
they stay "hidden" until the ancestor turns it off.
Because content-visibility: hidden also applies many containment values to the container,
it’s not always quite as usable as visibility: hidden would be,
but when its restrictions are acceptable,
it can be a more reliable,
more consistent way to hide an element’s contents.
content-visibility: auto is a more complex value than hidden;
rather than being similar to display: none,
it adaptively hides/displays an element’s contents
as they become relevant to the user.
It also doesn’t hide its skipped contents from the user agent,
so screen readers,
find-in-page,
and other tools can still interact with it.
It is best to think of it as an upgrade to containment:
if an author has a large amount of content to display
that will often be off-screen
(such as a long scrollable list),
and that content is okay with heavy containment,
they should consider using content-visibility: auto to apply all of the containments at once.
This also strongly hints to the user agent
that it’s acceptable to skip work on the contents
(possibly causing a small delay when they do come on-screen)
because it’s more important to have a large amount of content in the document
and most of it won’t be seen anyway.
Note:content-visibility: auto can thus be used instead of complicated "virtual list" techniques,
at least in many cases.
For example, on Twitter,
applying content-visibility: auto to the entire timeline
wouldn’t accomplish much—it’s always on-screen,
and so it will never skip its contents.
Instead, content-visibility should be applied to individual tweets,
allowing each of them to be skipped as they go off-screen.
Because content-visibility: auto imposes size containment when the element skips its contents,
if the element depends on its contents to determine its size
the layout of the page
(or at least, the scrollbar position)
can "jump around" as elements go off-screen and start skipping.
This can be fixed in a number of ways:
making the element fixed-size
carefully arranging a layout such as Grid
to size the element without depending on its contents
using contain-intrinsic-size: auto to automatically "snapshot" the exact size of the element
from the last time it was rendered,
before it was skipped (along with providing an estimate of the size
to be used before it’s rendered and can have its size snapshotted)
For example,
on Twitter,
the average tweet is approximately 200px tall,
so contain-intrinsic-size: auto 500px 200px will ensure that the scrollbar thumb is in approximately the correct size and position
even when preceding or following tweets are skipped,
while still allowing the tweets to size according to their contents
when they’re on-screen.
As long as the tweets have all been viewed at least once
(and haven’t changed size while they were skipped),
their sizes will be exactly correct while they’re skipped,
so the scrollbar thumb will be as well;
only freshly-loaded tweets
(such as those loaded at the top of the timeline
while you are scrolling further down)
will be forced to rely on the 200px height estimate.
The contentvisibilityautostatechange event is fired on an element with content-visibility: auto style when
the rendering state changes and the element either becomes or stops being relevant to the user.
Note that elements in content-visibility: auto subtrees remain semantically
relevant even for elements that skip its contents. This means that it is
inappropriate to use this signal to indefinitely skip DOM updates in the
subtree that is skipped. Instead, it should be used to deprioritize updates,
but ensure that the content remains semantically relevant and reasonably
up-to-date. This is particularly important for assistive technologies which
consume this content even when the ancestor is set to skip its contents.
4.5. Restrictions and Clarifications
From the perspective of an IntersectionObserver,
the skipped contents of an element
are never intersecting the intersection root.
This is true even if both the root and the target elements
are in the skipped contents.
From the perspective of a ResizeObserver,
the skipped contents of an element
never change their size.
If these elements become non-skipped later,
the resize observation will be delivered
if the new size differs from the last size used to notify the resize observer.
If an element starts or stops skipping its contents,
this change happens
after the requestAnimationFrame callbacks
of the frame that renders the effects of the change
have run.
Specifically, such changes will take effect between steps 13 and 14
of update the rendering step of the Processing Model
(between “run the animation frame callbacks”
and “run the update intersection observations steps”).
Determining the viewport intersection of the element
can be done with an internal version of an IntersectionObserver.
However, since the observations from this are dispatched at
step 14 of update the rendering,
any changes to the skipped (and thus painted) state
will not be visible to the user until the next frame’s processing.
For this reason, updating the skipped state,
including containment adjustments,
is deferred to that frame as well.
This ensures that script accessing,
for example,
the containment value of the element between these two events
(internal intersection observation and skipped state update)
will retrieve values consistent with current painted state
and not cause any forced layouts.
When an element first gains content-visibility: auto, it may or may not
be positioned on screen. The determination of this state and thus
determination of whether this element is skipped must happen in the
same frame. If it does not, then there is a possibility of producing blank
content in the element’s place since visibility check and skipped state
update would be deferred to the next frame.
Note: Once it’s scrolled into view,
the element will no longer skip its contents,
and so might not have size containment;
if this changes the element’s size,
it might not align in the viewport exactly as requested.
If an element is not available to user-agent features
(for example, if it is skipped due to a content-visibility: hidden ancestor),
then scrolling operations
must not scroll to it at all,
as if it did not have a layout box.
Note: Thus, unlike the previous point,
the element will be correctly sized and aligned in the viewport.
This is consistent with the order of the steps for the focus() method.
If an iframeskips its contents or is part of an element’s skipped contents,
the user agent should entirely skip the Update The Rendering step in the iframe’s event loop,
if possible.
While an element is skipped,
CSS transitions and animations on the element do not update:
New animations are not created even if newly-applied style would start one.
Existing animations do not advance in their timeline.
Running animations on the element do not end.
If script queries the style of a skipped element
(causing a style change event)
such that knowing the state of animations or transitions is required
to return correct information,
animation and transitions are sampled
according to the styles at the time of that style change event:
When an element stops being skipped,
animations and transitions are sampled
and then resume advancing on their timelines as normal from that point.
Note: Overall, this is similar to the behavior of transitions/animations
when a background tab is brought back to the foreground,
allowing user agents to skip as much unnecessary animation work as possible
without overly disrupting the animations when they become relevant again.
While an element is skipped,
it must not start any transitions,
even if a style change event affects its computed styles.
When an element stops being skipped,
it must not start any transitions
as a result of the style change event associated with it no longer being skipped.
Note: This is similar to an element switching from display:none to a non-none value—even though the styles are technically changing in that case
(from their initial values
to their "proper" values from the cascade),
no transitions are started.
Note: An element skipped for other reasons,
such as having a content-visibility: auto ancestor,
will still generate boxes as normal,
and might thus become un-skipped.
4.6. Accessibility Implications
If a user agent exposes some form of "accessibility tree",
akin to the DOM tree but specialized for accessibility use-cases such as screen-readers
(thus providing the positions/etc of elements relevant to accessibility APIs,
such as focusable elements),
then the skipped contents of content-visibility: hidden elements
must similarly be "skipped" (omitted) in the accessibility tree
(similar to how display: none elements
are omitted in all views of the document).
Skipped contents of content-visibility: auto elements
must not expose the fact that a user is interacting with the page via the accessibility tree,
rather than via rendering visually to the screen.
In particular, if a user agent uses content-visibility: auto to avoid doing layout and painting work on off-screen content
for displaying to the screen,
it must similarly avoid doing that work on off-screen content
for representing in the accessibility tree.
If this is not possible
(for example, if the user agent’s representation of a focusable element in the accessibility tree
requires knowledge of its exact position,
and thus requires a full layout to be done on it and surrounding contents),
then the user agent must omit the skipped contents from the accessibility tree entirely.
Note: This requirement is intended to protect users utilizing accessibility tooling
from being identified and profiled as such
via observation of timing channels;
if a user agent can skip significant amounts of work
when rendering visually,
but has to do all of the work when rendering to an accessibility tree,
then an author can tell how a user is interacting with the page
by observing the timing of layout operations.
4.7. Examples
<style>.sv{content-visibility:auto;min-height:50px;}</style><divclass=sv>
... some content goes here ...
</div>
The .sv element’s content-visibility: auto value
lets the user-agent manage whether the element is skipped.
Specifically when this element is near the viewport,
the user-agent will begin painting the element.
When the element moves away from the viewport,
it will stop being painted.
In addition, the user-agent should skip as much of the rendering work as possible
when the element is skipped.
<style>.sv{content-visibility:hidden;}</style><divclass=sv>
... some content goes here ...
</div>
In this case, the element is skipped regardless of viewport intersection.
This means that the only way to have the contents painted
is via script updating the value to remove content-visibility or change its value.
As before, the user-agent should skip as much of the rendering in the contents as possible.
An additional effect of skipping rendering
is that the layout state of the contents can be preserved by the user-agent,
so that removing the content-visibility property in the future
will cause the contents to be rendered quicker
than if they were hidden with display: none or similar.
<style>body{margin:0;}.sv{content-visibility:hidden;position:relative;left:10px;top:20px;}#child{position:relative;left:1px;top:2px;width:100px;height:200px;}</style><divid=targetclass=sv><divid=child></div>
... some other content goes here ...
</div><script>...// This will force rendering work, including layout,// if the UA previously avoided it.
target.firstElementChild.getBoundingClientRect();...</script>
Similarly to the last example, the element is skipped.
The user-agent should avoid as much rendering work as possible.
However, in this example, at some point script accesses a layout value in the element’s contents.
In this situation, the user-agent cannot avoid rendering work
and has to process any previously skipped rendering work
in order to return a correct value to the caller.
In this example, the result of getBoundingClientRect() is a rect positioned at (11, 22) with a size 100x200.
Note that repeated calls to the same layout value
should not cause any additional rendering work,
since the user-agent should retain the last updated rendering state.
Also note that this situation in which rendering work is required is not unique.
There may be other situations in which the user-agent cannot avoid rendering work.
5. Privacy Considerations
There are no known privacy impacts of the features in this specification.
6. Security Considerations
There are no known security impacts of the features in this specification.
Like any other CSS specification, it affects the rendering of the document,
but does not introduce any special ability to present content in a misleading way
that was not previously available through other CSS modules
and that isn’t inherent to the act of formatting the document.
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.
Tests
Tests relating to the content of this specification
may be documented in “Tests” blocks like this one.
Any such block is non-normative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
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
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.
In general, the relationship between an element’s inline size
and it’s block size
is unpredictable and non-monotonic,
with the block size capable of shifting up and down arbitrarily
as the inline size is changed.
Infinite cycles are prevented
by ensuring that layout does not revert to a previous (known-problematic) state,
even if a naive analysis of the constraints would allow for such;
in other words, layout always “moves forward”.
We believe that current CSS layout specifications incorporate such rules,
but to the extent that they don’t,
please inform the CSSWG so that these errors can be corrected. ↵
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