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<pre class=metadata>
Title: CSS Containment Module Level 1
Level: 1
Shortname: css-contain
Status: REC
Date: 2022-10-25
Deadline: 2023-01-10
Prepare for TR: yes
Implementation Report: https://drafts.csswg.org/css-contain-1/implementation-report-2022-09
Work Status: completed
Group: csswg
ED: https://drafts.csswg.org/css-contain-1/
TR: https://www.w3.org/TR/css-contain-1/
Previous Version: https://www.w3.org/TR/2020/REC-css-contain-1-20201222/
Previous Version: https://www.w3.org/TR/2019/REC-css-contain-1-20191121/
Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
Editor: Florian Rivoal, On behalf of Bloomberg, https://florian.rivoal.net/, w3cid 43241
Abstract: 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.
Status Text: <p>This document was published by the CSS Working Group as a Recommendation using the <a href="https://www.w3.org/2021/Process-20211102/#recs-and-notes">Recommendation track</a>. It includes <a href="https://www.w3.org/2021/Process-20211102/#proposed-corrections">proposed corrections</a>.</p><p class="correction proposed">Proposed corrections are marked in the document.</p><p>W3C recommends the wide deployment of this specification as a standard for the Web.</p>
WPT Path Prefix: css/css-contain/
Include MDN Panels: no
</pre>
<pre class="anchors">
spec: css2; urlPrefix: https://www.w3.org/TR/CSS2/
type: property; url: visudet.html#propdef-vertical-align; text: vertical-align
type: dfn; url: box.html#padding-edge; text: padding edge
spec: css-backgrounds-3;
type: dfn; url: https://drafts.csswg.org/css-backgrounds-3/#corner-clipping; text: corner clipping
</pre>
<pre class=link-defaults>
spec:css21; type:dfn; text:stacking context
spec:css-display-3; type:property; text:display
spec:css-ui-3; type:property; text:text-overflow
spec:css-grid-1; type:property; text:grid
spec:css-break-3; type:dfn; text:forced break
spec:css-break-3; type:dfn; text:fragmentation
spec:css-break-3; type:dfn; text:fragmentation container
spec:css-break-3; type:dfn; text:fragmentation context
spec:css-break-3; type:dfn; text:fragmented flow
spec:css21; type:value; property: height; text:auto
spec:css-sizing-4; type:property; text:aspect-ratio
spec:css-sizing-3; type:value; for:width; text:min-content
spec:css-sizing-3; type:value; for:width; text:max-content
</pre>
<h2 id='intro'>
Introduction</h2>
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.
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> 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 <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h2 id='contain-property'>
Strong Containment: the 'contain' property</h2>
<wpt title="The following tests are crash tests
that relate to general usage
of this feature
but are not particularly tied to any particular normative statement.">
contain-chrome-thcrash-001.html
contain-crash.html
contain-flexbox-outline.html
crashtests/contain-nested-crash-001.html
crashtests/contain-nested-crash-002.html
crashtests/contain-nested-crash-003.html
</wpt>
<div class="correction proposed" id=c1>
<span class=marker>Proposed Correction 1:</span>
This makes a minor adjustment
to the way the computed value of the 'contain' property is determined:
the shortcut values (''strict'' and ''content''),
instead of computing to themselves,
compute to the corresponding keywords.
Given that the effect is identical,
this allows implementations not to store the precise syntax through which this was achieved.
Also, thanks to the shortest-serialization principle,
this makes this unimportant difference non observable via serialization.
<details>
<summary>This change has tests</summary>
A <a href="https://github.com/web-platform-tests/wpt/commit/98c4e2d52402156bfafcd1466985b5336f6c7240">test for this change</a> has been added to <abbr title="Web Platform Tests">WPT</abbr>.
The results can be viewed at <a href="https://wpt.fyi/results/css/css-contain/parsing/contain-computed.html">wpt.fyi</a>.
Note that a few subtests of this test
(those involving the <code>style</code> or <code>inline-size</code> values)
only relate to later levels of this specification.
</details>
</div>
<pre class='propdef'>
Name: contain
Value: none | strict | content | [ size || layout || paint ]
Initial: none
Inherited: no
Applies to: See <a href="#contain-applies">below</a>
Computed value: <del cite=#c1 class=proposed>specified keyword(s)</del><ins cite=#c1 class=proposed>the keyword ''contain/none'' or one or more of ''size'', ''layout'', ''paint''</ins>
Animation type: not animatable
</pre>
<wpt>
contain-animation-001.html
inheritance.html
parsing/contain-computed.html
parsing/contain-invalid.html
parsing/contain-valid.html
parsing/contain-computed-children.html
contain-layout-size-003.html
contain-paint-size-001.html
contain-paint-size-002.html
contain-paint-size-003.html
</wpt>
<p class=all-media>User agents are expected to support this property on all media, including non-visual ones.</p>
The 'contain' property allows an author to indicate that an element and its contents are,
as much as possible,
<em>independent</em> 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.
<dl dfn-type=value dfn-for=contain>
<dt><dfn>none</dfn>
<dd>
This value indicates that the property has no effect.
The element renders as normal,
with no containment effects applied.
<dt><dfn>strict</dfn>
<dd>
This value <ins cite=#c1>computes to ''size layout paint'',
and thus </ins>turns on all forms of <a>containment</a> for the element.
<del cite=#c1>In other words, it behaves the same as ''contain: size layout paint;''.</del>.
<wpt>
contain-strict-001.html
contain-strict-002.html
contain-strict-003.html
contain-flexbox-outline.html
</wpt>
<dt><dfn>content</dfn>
<dd>
This value <ins cite=#c1>computes to ''layout paint'',
and thus </ins>turns on all forms of <a>containment</a> <em>except</em> <a>size containment</a> for the element.
<del cite=#c1>In other words, it behaves the same as ''contain: layout paint;''.</del>.
<wpt>
contain-content-001.html
contain-content-002.html
contain-content-003.html
contain-content-004.html
</wpt>
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 <a>size containment</a>,
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.
<dt><dfn>size</dfn>
<dd>
The value turns on <a>size containment</a> for the element.
This ensures that the [=size containment box|containment box=] can be laid out
without needing to examine its descendants.
<wpt>
contain-size-001.html
contain-size-002.html
contain-size-003.html
contain-size-004.html
contain-size-005.html
contain-size-006.html
contain-size-007.html
contain-size-008.html
contain-size-009.html
contain-size-010.html
contain-size-011.html
contain-size-012.html
contain-size-012b.html
contain-size-013.html
contain-size-021.html
contain-size-023.html
contain-size-025.html
contain-size-027.html
contain-size-041.html
contain-size-042.html
contain-size-051.html
contain-size-052.html
contain-size-056.html
contain-size-061.html
contain-size-062.html
contain-size-063.html
contain-size-baseline-001.html
contain-size-borders.html
contain-size-breaks-001.html
contain-size-button-001.html
contain-size-fieldset-001.html
contain-size-fieldset-002.html
contain-size-flexbox-001.html
contain-size-flexbox-002.html
contain-size-grid-001.html
contain-size-grid-002.html
contain-size-grid-003.html
contain-size-grid-004.html
contain-size-grid-005.html
contain-size-grid-006.html
contain-size-monolithic-001.html
contain-size-monolithic-002.html
contain-size-multicol-001.html
contain-size-multicol-as-flex-item.html
contain-size-replaced-001.html
contain-size-replaced-002.html
contain-size-replaced-003a.html
contain-size-replaced-003b.html
contain-size-replaced-003c.html
contain-size-replaced-004.html
contain-size-replaced-005.html
contain-size-replaced-006.html
contain-size-replaced-007.html
contain-size-select-001.html
contain-size-select-002.html
contain-size-scrollbars-001.html
contain-size-scrollbars-002.html
contain-size-scrollbars-003.html
contain-size-scrollbars-004.html
contain-layout-size-003.html
contain-paint-size-001.html
contain-paint-size-002.html
contain-paint-size-003.html
contain-size-removed.html
</wpt>
<dt><dfn>layout</dfn>
<dd>
This value turns on <a>layout containment</a> for the element.
This ensures that the [=layout containment box|containment box=] is <em>totally opaque</em> for layout purposes;
nothing outside can affect its internal layout,
and vice versa.
<wpt>
contain-layout-001.html
contain-layout-002.html
contain-layout-003.html
contain-layout-004.html
contain-layout-005.html
contain-layout-006.html
contain-layout-007.html
contain-layout-009.html
contain-layout-010.html
contain-layout-011.html
contain-layout-012.html
contain-layout-013.html
contain-layout-014.html
contain-layout-016.html
contain-layout-017.html
contain-layout-018.html
contain-layout-019.html
contain-layout-020.html
contain-layout-021.html
contain-layout-baseline-001.html
contain-layout-baseline-002.html
contain-layout-baseline-003.html
contain-layout-baseline-004.html
contain-layout-baseline-005.html
contain-layout-breaks-001.html
contain-layout-breaks-002.html
contain-layout-button-001.html
contain-layout-cell-001.html
contain-layout-cell-002.html
contain-layout-flexbox-001.html
contain-layout-grid-001.html
contain-layout-ifc-022.html
contain-layout-independent-formatting-context-001.html
contain-layout-independent-formatting-context-002.html
contain-layout-independent-formatting-context-003.html
contain-layout-ink-overflow-013.html
contain-layout-ink-overflow-014.html
contain-layout-ink-overflow-015.html
contain-layout-ink-overflow-016.html
contain-layout-ink-overflow-017.html
contain-layout-ink-overflow-018.html
contain-layout-ink-overflow-019.html
contain-layout-ink-overflow-020.html
contain-layout-size-003.html
contain-subgrid-001.html
</wpt>
<dt><dfn>paint</dfn>
<dd>
This value turns on <a>paint containment</a> for the element.
This ensures that the descendants of the [=paint containment box|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.
<wpt>
contain-paint-001.html
contain-paint-002.html
contain-paint-004.html
contain-paint-005.html
contain-paint-006.html
contain-paint-007.html
contain-paint-008.html
contain-paint-009.html
contain-paint-010.html
contain-paint-011.html
contain-paint-012.html
contain-paint-014.html
contain-paint-015.html
contain-paint-016.html
contain-paint-017.html
contain-paint-018.html
contain-paint-019.html
contain-paint-020.html
contain-paint-021.html
contain-paint-022.html
contain-paint-023.html
contain-paint-024.html
contain-paint-025.html
contain-paint-047.html
contain-paint-048.html
contain-paint-baseline-001.html
contain-paint-cell-001.html
contain-paint-cell-002.html
contain-paint-clip-011.html
contain-paint-clip-012.html
contain-paint-clip-013.html
contain-paint-clip-014.html
contain-paint-clip-015.html
contain-paint-clip-016.html
contain-paint-clip-017.html
contain-paint-clip-018.html
contain-paint-clip-019.html
contain-paint-ifc-011.html
contain-paint-independent-formatting-context-001.html
contain-paint-independent-formatting-context-002.html
contain-paint-independent-formatting-context-003.html
contain-paint-size-001.html
contain-paint-size-002.html
contain-paint-size-003.html
contain-paint-table-001.html
contain-paint-table-002.html
contain-subgrid-001.html
contain-paint-change-opacity.html
</wpt>
</dl>
<span id="contain-applies">This property generally applies to all elements (including [[css-pseudo-4#generated-content]])</span>,
although some types of containment have no effect on some elements,
as detailed in [[#containment-types]].
In addition, in the case of [[SVG2]],
the 'contain' property only applies to <{svg}> elements that have an associated CSS layout box.
<div class='example'>
'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:
<pre><code highlight=markup>
<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>
</code></pre>
There are probably a <em>lot</em> 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.
</div>
<div class="correction proposed" id=c3>
<span class=marker>Proposed Correction 3:</span>
The CSS Working Group had forgotten to consider the effects of containment
on the HTML <{html}> and <{body}> elements,
particularly in consideration of the fact that
for legacy reasons,
some properties can propagate outwards from the <{body}> element.
This proposed text addresses this oversight.
<blockquote>
Additionally, when the [=used value=] of the 'contain' property
on either the HTML <{html}> or <{body}> elements
is anything other than ''contain/none'',
propagation of properties
from the <{body}> element
to the [=initial containing block=], the viewport, or the [=canvas background=],
is disabled.
Notably, this affects:
* 'writing-mode', 'direction', and 'text-orientation' (see [[CSS-WRITING-MODES-3#principal-flow]])
* 'overflow' and its longhands (see [[CSS-OVERFLOW-3#overflow-propagation]])
* 'background' and its longhands (see [[CSS-BACKGROUNDS-3#body-background]])
<wpt>
contain-body-bg-001.html
contain-body-bg-002.html
contain-body-bg-003.html
contain-body-bg-004.html
contain-body-dir-001.html
contain-body-dir-002.html
contain-body-dir-003.html
contain-body-dir-004.html
contain-body-overflow-001.html
contain-body-overflow-002.html
contain-body-overflow-003.html
contain-body-overflow-004.html
contain-body-t-o-001.html
contain-body-t-o-002.html
contain-body-t-o-003.html
contain-body-t-o-004.html
contain-body-w-m-001.html
contain-body-w-m-002.html
contain-body-w-m-003.html
contain-body-w-m-004.html
contain-html-bg-001.html
contain-html-bg-002.html
contain-html-bg-003.html
contain-html-bg-004.html
contain-html-dir-001.html
contain-html-dir-002.html
contain-html-dir-003.html
contain-html-dir-004.html
contain-html-overflow-001.html
contain-html-overflow-002.html
contain-html-overflow-003.html
contain-html-overflow-004.html
contain-html-t-o-001.html
contain-html-t-o-002.html
contain-html-t-o-003.html
contain-html-t-o-004.html
contain-html-w-m-001.html
contain-html-w-m-002.html
contain-html-w-m-003.html
contain-html-w-m-004.html
</wpt>
Note: Propagation
to the [=initial containing block=], the viewport, or the [=canvas background=],
of properties set on the <{html}> element itself
is unaffected.
</blockquote>
<details>
<summary>This change has tests</summary>
Tests to cover these new requirements have been added to <abbr title="Web Platform Tests">WPT</abbr>.
The results of these tests can be seen at the <a href="https://wpt.fyi/">Web-Platform-Tests dashboard</a>:
<ol>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-bg-001.html>contain-body-bg-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-bg-002.html>contain-body-bg-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-bg-003.html>contain-body-bg-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-bg-004.html>contain-body-bg-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-dir-001.html>contain-body-dir-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-dir-002.html>contain-body-dir-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-dir-003.html>contain-body-dir-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-dir-004.html>contain-body-dir-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-overflow-001.html>contain-body-overflow-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-overflow-002.html>contain-body-overflow-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-overflow-003.html>contain-body-overflow-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-overflow-004.html>contain-body-overflow-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-t-o-001.html>contain-body-t-o-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-t-o-002.html>contain-body-t-o-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-t-o-003.html>contain-body-t-o-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-t-o-004.html>contain-body-t-o-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-w-m-001.html>contain-body-w-m-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-w-m-002.html>contain-body-w-m-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-w-m-003.html>contain-body-w-m-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-body-w-m-004.html>contain-body-w-m-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-bg-001.html>contain-html-bg-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-bg-002.html>contain-html-bg-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-bg-003.html>contain-html-bg-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-bg-004.html>contain-html-bg-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-dir-001.html>contain-html-dir-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-dir-002.html>contain-html-dir-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-dir-003.html>contain-html-dir-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-dir-004.html>contain-html-dir-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-overflow-001.html>contain-html-overflow-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-overflow-002.html>contain-html-overflow-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-overflow-003.html>contain-html-overflow-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-overflow-004.html>contain-html-overflow-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-t-o-001.html>contain-html-t-o-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-t-o-002.html>contain-html-t-o-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-t-o-003.html>contain-html-t-o-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-t-o-004.html>contain-html-t-o-004.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-w-m-001.html>contain-html-w-m-001.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-w-m-002.html>contain-html-w-m-002.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-w-m-003.html>contain-html-w-m-003.html</a>
<li><a href=https://wpt.fyi/results/css/css-contain/contain-html-w-m-004.html>contain-html-w-m-004.html</a>
</ol>
</details>
</div>
<h2 id='containment-types'>
Types of Containment</h2>
There are several varieties of <dfn export>containment</dfn> that an element can be subject to,
restricting the effects that its descendants can have on the rest of the page in various ways.
<a>Containment</a> 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.
Advisement: 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.
<h3 id='containment-size'>
Size Containment</h3>
Giving an element <dfn export>size containment</dfn>
makes its [=principal box=] a <dfn>size containment box</dfn>
and has the following effects:
<ol>
<li>
When calculating the size of [=size containment box=],
it must be treated as having no contents.
<wpt>
contain-size-021.html
contain-size-023.html
contain-size-025.html
contain-size-027.html
contain-size-061.html
contain-size-062.html
contain-size-063.html
contain-size-borders.html
contain-size-fieldset-001.html
contain-size-fieldset-002.html
contain-size-select-001.html
contain-size-select-002.html
contain-size-scrollbars-001.html
contain-size-scrollbars-002.html
contain-size-scrollbars-003.html
contain-size-scrollbars-004.html
contain-size-button-001.html
contain-size-flexbox-001.html
contain-size-flexbox-002.html
contain-size-grid-001.html
contain-size-013.html
contain-size-041.html
contain-size-042.html
contain-size-replaced-001.html
contain-size-replaced-002.html
contain-size-replaced-003a.html
contain-size-replaced-003b.html
contain-size-replaced-003c.html
contain-size-replaced-004.html
contain-size-replaced-005.html
contain-size-replaced-006.html
contain-size-block-001.html
contain-size-block-002.html
contain-size-block-003.html
contain-size-block-004.html
contain-size-button-002.html
contain-size-fieldset-003.html
</wpt>
<wpt pathprefix=css/css-sizing/aspect-ratio/>
replaced-element-023.html
replaced-element-025.html
replaced-element-027.html
</wpt>
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.
<wpt>
contain-size-grid-002.html
contain-size-grid-003.html
contain-size-multicol-001.html
contain-size-multicol-as-flex-item.html
</wpt>
Then, its contents must then be laid out into the [=size containment box|containment box=]'s resolved size.
Note: [=Size containment=] does not suppress baseline alignment.
See [=layout containment=] for that.
<wpt>
contain-size-baseline-001.html
</wpt>
<div class="correction proposed" id=c2>
<span class=marker>Proposed Correction 2:</span>
The way the two normative sentences above are written was somewhat ambiguous,
causing implementers to have some doubts about the precise intended effects
in certain cases.
In order to clarify what is meant without changing the intended behavior,
The CSSWG suggest replacing this first item in the list with the following two
(the existing notes remain the same):
<blockquote>
<ol>
<li>
The [=intrinsic sizes=] of the [=size containment box=]
are determined as if the element had no content,
following the same logic as when [=sizing as if empty=].
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 <span class=informative>[=grid tracks=]</span> into which a size contained item is placed,
or if [=fit-content sizing=] the containment box's parent.
<wpt>
contain-size-013.html
contain-size-041.html
contain-size-042.html
contain-size-replaced-001.html
contain-size-replaced-002.html
contain-size-replaced-003a.html
contain-size-replaced-003b.html
contain-size-replaced-003c.html
contain-size-replaced-004.html
contain-size-replaced-005.html
contain-size-replaced-006.html
contain-size-block-001.html
contain-size-block-002.html
contain-size-block-003.html
contain-size-block-004.html
contain-size-button-002.html
contain-size-fieldset-003.html
contain-size-flex-001.html
contain-size-grid-005.html
contain-size-grid-006.html
contain-size-inline-block-001.html
contain-size-inline-block-002.html
contain-size-inline-block-003.html
contain-size-inline-block-004.html
contain-size-inline-flex-001.html
contain-size-multicol-002.html
contain-size-multicol-003.html
contain-size-select-elem-001.html
contain-size-select-elem-002.html
contain-size-select-elem-003.html
contain-size-select-elem-004.html
contain-size-select-elem-005.html
</wpt>
<li>
Laying out a [=size containment box=] and its content is conceptually done in two phases:
<dl>
<dt><dfn>Sizing as if empty</dfn>
<dd>
The [=used value|used=] 'width' and 'height' of the [=size containment box|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''.
<wpt>
contain-size-021.html
contain-size-023.html
contain-size-025.html
contain-size-027.html
contain-size-061.html
contain-size-062.html
contain-size-063.html
contain-size-borders.html
contain-size-fieldset-001.html
contain-size-fieldset-002.html
contain-size-select-001.html
contain-size-select-002.html
contain-size-scrollbars-001.html
contain-size-scrollbars-002.html
contain-size-scrollbars-003.html
contain-size-scrollbars-004.html
contain-size-button-001.html
contain-size-flexbox-001.html
contain-size-flexbox-002.html
contain-size-grid-001.html
</wpt>
<a>Replaced elements</a> must be treated as having a [=natural dimensions|natural=] width and height of 0
and no [=natural aspect ratio=].
Note: Size containment only suppresses the [=natural aspect ratio=],
so properties like 'aspect-ratio' which affect that [=preferred aspect ratio=] directly
are honored.
<wpt>
contain-size-013.html
contain-size-041.html
contain-size-042.html
contain-size-replaced-001.html
contain-size-replaced-002.html
contain-size-replaced-003a.html
contain-size-replaced-003b.html
contain-size-replaced-003c.html
contain-size-replaced-004.html
contain-size-replaced-005.html
</wpt>
<wpt pathprefix=css/css-sizing/aspect-ratio/>
replaced-element-023.html
replaced-element-025.html
replaced-element-027.html
</wpt>
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.
<wpt>
contain-size-replaced-006.html
</wpt>
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.
<wpt>
contain-size-grid-002.html
contain-size-grid-003.html
contain-size-multicol-001.html
contain-size-multicol-as-flex-item.html
</wpt>
<dt><dfn>Laying out in-place</dfn>
<dd>
The [=size containment box|containment box=]'s content
(including any pseudo-elements)
must then be laid out into
the now fixed-size [=size containment box|containment box=] normally.
<wpt>
contain-size-064.html
<wpt>
</dl>
Note: [=Size containment=] does not suppress baseline alignment.
See [=layout containment=] for that.
<wpt>
contain-size-baseline-001.html
contain-size-fieldset-004.html
contain-size-inline-block-001.html
contain-size-inline-block-002.html
contain-size-inline-block-003.html
contain-size-inline-block-004.html
contain-size-inline-flex-001.html
</wpt>
</ol>
</blockquote>
<details>
<summary>This change has tests</summary>
All statements in this more detailed write-up have tests in <abbr title="Web Platform Tests">WPT</abbr>.
The results of these tests can be seen at the <a href="https://wpt.fyi/">Web-Platform-Tests dashboard</a>:
<ol>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-013.html">contain-size-013.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-021.html">contain-size-021.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-023.html">contain-size-023.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-025.html">contain-size-025.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-027.html">contain-size-027.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-041.html">contain-size-041.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-042.html">contain-size-042.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-061.html">contain-size-061.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-062.html">contain-size-062.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-063.html">contain-size-063.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-064.html">contain-size-064.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-borders.html">contain-size-borders.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-button-001.html">contain-size-button-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-fieldset-001.html">contain-size-fieldset-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-fieldset-002.html">contain-size-fieldset-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-flexbox-001.html">contain-size-flexbox-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-flexbox-002.html">contain-size-flexbox-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-grid-001.html">contain-size-grid-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-grid-002.html">contain-size-grid-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-grid-003.html">contain-size-grid-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-multicol-001.html">contain-size-multicol-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-multicol-as-flex-item.html">contain-size-multicol-as-flex-item.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-001.html">contain-size-replaced-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-002.html">contain-size-replaced-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-003a.html">contain-size-replaced-003a.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-003b.html">contain-size-replaced-003b.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-003c.html">contain-size-replaced-003c.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-004.html">contain-size-replaced-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-005.html">contain-size-replaced-005.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-replaced-006.html">contain-size-replaced-006.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-scrollbars-001.html">contain-size-scrollbars-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-scrollbars-002.html">contain-size-scrollbars-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-scrollbars-003.html">contain-size-scrollbars-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-scrollbars-004.html">contain-size-scrollbars-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-001.html">contain-size-select-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-002.html">contain-size-select-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-sizing/aspect-ratio/replaced-element-023.html">replaced-element-023.html</a>
<li><a href="https://wpt.fyi/results/css/css-sizing/aspect-ratio/replaced-element-025.html">replaced-element-025.html</a>
<li><a href="https://wpt.fyi/results/css/css-sizing/aspect-ratio/replaced-element-027.html">replaced-element-027.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-block-001.html">contain-size-block-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-block-002.html">contain-size-block-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-block-003.html">contain-size-block-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-block-004.html">contain-size-block-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-button-002.html">contain-size-button-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-fieldset-003.html">contain-size-fieldset-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-flex-001.html">contain-size-flex-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-grid-005.html">contain-size-grid-005.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-grid-006.html">contain-size-grid-006.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-001.html">contain-size-inline-block-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-002.html">contain-size-inline-block-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-003.html">contain-size-inline-block-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-004.html">contain-size-inline-block-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-flex-001.html">contain-size-inline-flex-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-multicol-002.html">contain-size-multicol-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-multicol-003.html">contain-size-multicol-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-elem-001.html">contain-size-select-elem-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-elem-002.html">contain-size-select-elem-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-elem-003.html">contain-size-select-elem-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-elem-004.html">contain-size-select-elem-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-select-elem-005.html">contain-size-select-elem-005.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-fieldset-004.html">contain-size-fieldset-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-001.html">contain-size-inline-block-001.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-002.html">contain-size-inline-block-002.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-003.html">contain-size-inline-block-003.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-block-004.html">contain-size-inline-block-004.html</a>
<li><a href="https://wpt.fyi/results/css/css-contain/contain-size-inline-flex-001.html">contain-size-inline-flex-001.html</a>
</ol>
</details>
Also, note the insertion of a parenthetical at the very end of [[#containment-size-opt]],
referencing part of this clarified text.
</div>
<li>
[=Size containment boxes=] are <a spec=css-break-3>monolithic</a> (See [[CSS-BREAK-3#possible-breaks]]).
<wpt>
contain-size-breaks-001.html
contain-size-monolithic-001.html
contain-size-monolithic-002.html
contain-size-multicol-004.html
</wpt>
</ol>
<div class=example>
Given the following markup and style, the image would be sized to 100px by 100px,
as the aspect ratio set by the 'aspect-ratio' property takes effect.
<pre><code class=lang-css>
img {
width: 100px;
aspect-ratio: 1/1;
contain: size;
}
</code><code class=lang-html>
<img src="https://www.example.com/300x100.jpg">
</code></pre>
If the 'aspect-ratio' property had not been declared,
the image would have been 100px by 0px,
as its [=natural aspect ratio=] is suppressed,
and its [=natural height=] is treated as 0.
</div>
However, giving an element [=size containment=]
has no effect if any of the following are true:
* if the element does not generate a <a>principal box</a>
(as is the case with ''display: contents'' or ''display: none'')
* if its [=inner display type=] is ''display/table''
<wpt>
contain-size-012.html
contain-size-012b.html
</wpt>
* if its [=principal box=] is
an <a spec="css-display-3">internal table box</a>
<wpt>
contain-size-006.html
contain-size-007.html
contain-size-008.html
contain-size-009.html
contain-size-010.html
contain-size-051.html
contain-size-052.html
</wpt>
* if its [=principal box=] is
an <a spec="css-display-3">internal ruby box</a>
or a <a spec="css-display-3" lt="atomic inline">non-atomic</a> <a spec="css-display-3">inline-level</a> box
<wpt>
contain-size-002.html
contain-size-003.html
contain-size-004.html
contain-size-005.html
contain-size-001.html
</wpt>
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.
<wpt>
contain-size-011.html
contain-size-056.html
contain-size-table-caption-001.html
</wpt>
<h4 id='containment-size-opt' class="no-toc">
Possible Size-Containment Optimizations</h4>
<em>This section is non-normative.</em>
By itself, <a>size containment</a> does not offer much optimization opportunity.
Its primary benefit on its own is that tools which want to lay out the [=size containment box|containment box=]'s contents
based on the [=size containment box|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 [=size containment box|containment box=]
causes the [=size containment box|containment box=]'s size to change as well,
possibly triggering <em>further</em> changes in how the child sizes itself
and possibly thus more changes to the [=size containment box|containment box=]'s size,
ad infinitum.
When paired with <a>layout containment</a>, though,
possible optimizations that can be enabled include (but are not limited to):
1. When the style or contents of a descendant of the [=size containment box|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 [=size containment box|containment box=].
2. When laying out the page,
if the [=size containment box|containment box=] is off-screen or obscured,
the layout of its contents <ins cite=#c2 class=proposed>(i.e. "[=laying out in-place=]") </ins>can be delayed or done at a lower priority.
<h3 id='containment-layout'>
Layout Containment</h3>
Giving an element <dfn export>layout containment</dfn>
makes its [=principal box=] a <dfn>layout containment box</dfn>
and has the following effects:
1. The [=layout containment box=] [=establishes an independent formatting context=].
<wpt>
contain-layout-ifc-022.html
contain-layout-independent-formatting-context-001.html
contain-layout-independent-formatting-context-002.html
contain-subgrid-001.html
contain-layout-formatting-context-float-001.html
contain-layout-formatting-context-margin-001.html
</wpt>
2. 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 box=]