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annotated-2022-09-06.html
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<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<title>CSS Containment Module Level 1</title>
<meta content="ED" name="w3c-status">
<link href="https://www.w3.org/StyleSheets/TR/2021/W3C-ED" rel="stylesheet">
<link href="https://www.w3.org/2008/site/images/favicon.ico" rel="icon">
<meta content="Bikeshed version 2e3c4c68b, updated Fri Jan 14 14:49:00 2022 -0800" name="generator">
<link href="https://www.w3.org/TR/css-contain-1/" rel="canonical">
<link href="https://drafts.csswg.org/csslogo.ico" rel="icon">
<meta content="f502eec13e08f5f359615c321f5369d5d6eb3648" name="document-revision">
<script defer src="https://test.csswg.org/harness/annotate.js#!css-contain-1_dev/css-contain-1"></script>
<style>/* style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
color: var(--a-normal-text);
font-size: inherit;
font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
content: "‘";
}
.css::after, .property::after, .descriptor::after {
content: "’";
}
.property, .descriptor {
/* Don't wrap property and descriptor names */
white-space: nowrap;
}
.type { /* CSS value <type> */
font-style: italic;
}
pre .property::before, pre .property::after {
content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
content: "’";
}
[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
content: "";
}
[data-link-type=element],
[data-link-type=element-attr] {
font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after { content: ">" }
[data-link-type=biblio] {
white-space: pre;
}</style>
<style>/* style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
color-scheme: light dark;
--text: black;
--bg: white;
--unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);
--logo-bg: #1a5e9a;
--logo-active-bg: #c00;
--logo-text: white;
--tocnav-normal-text: #707070;
--tocnav-normal-bg: var(--bg);
--tocnav-hover-text: var(--tocnav-normal-text);
--tocnav-hover-bg: #f8f8f8;
--tocnav-active-text: #c00;
--tocnav-active-bg: var(--tocnav-normal-bg);
--tocsidebar-text: var(--text);
--tocsidebar-bg: #f7f8f9;
--tocsidebar-shadow: rgba(0,0,0,.1);
--tocsidebar-heading-text: hsla(203,20%,40%,.7);
--toclink-text: var(--text);
--toclink-underline: #3980b5;
--toclink-visited-text: var(--toclink-text);
--toclink-visited-underline: #054572;
--heading-text: #005a9c;
--hr-text: var(--text);
--algo-border: #def;
--del-text: red;
--del-bg: transparent;
--ins-text: #080;
--ins-bg: transparent;
--a-normal-text: #034575;
--a-normal-underline: #bbb;
--a-visited-text: var(--a-normal-text);
--a-visited-underline: #707070;
--a-hover-bg: rgba(75%, 75%, 75%, .25);
--a-active-text: #c00;
--a-active-underline: #c00;
--blockquote-border: silver;
--blockquote-bg: transparent;
--blockquote-text: currentcolor;
--issue-border: #e05252;
--issue-bg: #fbe9e9;
--issue-text: var(--text);
--issueheading-text: #831616;
--example-border: #e0cb52;
--example-bg: #fcfaee;
--example-text: var(--text);
--exampleheading-text: #574b0f;
--note-border: #52e052;
--note-bg: #e9fbe9;
--note-text: var(--text);
--noteheading-text: hsl(120, 70%, 30%);
--notesummary-underline: silver;
--assertion-border: #aaa;
--assertion-bg: #eee;
--assertion-text: black;
--advisement-border: orange;
--advisement-bg: #fec;
--advisement-text: var(--text);
--advisementheading-text: #b35f00;
--warning-border: red;
--warning-bg: hsla(40,100%,50%,0.95);
--warning-text: var(--text);
--amendment-border: #330099;
--amendment-bg: #F5F0FF;
--amendment-text: var(--text);
--amendmentheading-text: #220066;
--def-border: #8ccbf2;
--def-bg: #def;
--def-text: var(--text);
--defrow-border: #bbd7e9;
--datacell-border: silver;
--indexinfo-text: #707070;
--indextable-hover-text: black;
--indextable-hover-bg: #f7f8f9;
--outdatedspec-bg: rgba(0, 0, 0, .5);
--outdatedspec-text: black;
--outdated-bg: maroon;
--outdated-text: white;
--outdated-shadow: red;
--editedrec-bg: darkorange;
}</style>
<style>/* style-counters */
body {
counter-reset: example figure issue;
}
.issue {
counter-increment: issue;
}
.issue:not(.no-marker)::before {
content: "Issue " counter(issue);
}
.example {
counter-increment: example;
}
.example:not(.no-marker)::before {
content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
content: "Invalid Example" counter(example);
}
figcaption {
counter-increment: figure;
}
figcaption:not(.no-marker)::before {
content: "Figure " counter(figure) " ";
}</style>
<style>/* style-dfn-panel */
:root {
--dfnpanel-bg: #ddd;
--dfnpanel-text: var(--text);
}
.dfn-panel {
position: absolute;
z-index: 35;
height: auto;
width: -webkit-fit-content;
width: fit-content;
max-width: 300px;
max-height: 500px;
overflow: auto;
padding: 0.5em 0.75em;
font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
background: var(--dfnpanel-bg);
color: var(--dfnpanel-text);
border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: var(--dfnpanel-text); }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0; }
.dfn-panel li { list-style: inside; }
.dfn-panel.activated {
display: inline-block;
position: fixed;
left: .5em;
bottom: 2em;
margin: 0 auto;
max-width: calc(100vw - 1.5em - .4em - .5em);
max-height: 30vh;
}
.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-issues */
a[href].issue-return {
float: right;
float: inline-end;
color: var(--issueheading-text);
font-weight: bold;
text-decoration: none;
}
</style>
<style>/* style-md-lists */
/* This is a weird hack for me not yet following the commonmark spec
regarding paragraph and lists. */
[data-md] > :first-child {
margin-top: 0;
}
[data-md] > :last-child {
margin-bottom: 0;
}</style>
<style>/* style-selflinks */
:root {
--selflink-text: white;
--selflink-bg: gray;
--selflink-hover-text: black;
}
.heading, .issue, .note, .example, li, dt {
position: relative;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
}
a.self-link:hover {
opacity: 1;
}
.heading > a.self-link {
font-size: 83%;
}
li > a.self-link {
left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
top: auto;
left: auto;
opacity: 0;
width: 1.5em;
height: 1.5em;
background: var(--selflink-bg);
color: var(--selflink-text);
font-style: normal;
transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
opacity: 1;
}
dfn > a.self-link:hover {
color: var(--selflink-hover-text);
}
a.self-link::before { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before { content: "#"; }
</style>
<style>/* style-syntax-highlighting */
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
.highlight:not(.idl) { background: rgba(0, 0, 0, .03); }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
<style>/* style-wpt */
:root {
--wpt-border: hsl(0, 0%, 60%);
--wpt-bg: hsl(0, 0%, 95%);
--wpt-text: var(--text);
--wptheading-text: hsl(0, 0%, 30%);
}
@media (prefers-color-scheme: dark) {
:root {
--wpt-border: hsl(0, 0%, 30%);
--wpt-bg: var(--borderedblock-bg);
--wpt-text: var(--text);
--wptheading-text: hsl(0, 0%, 60%);
}
}
.wpt-tests-block {
list-style: none;
border-left: .5em solid var(--wpt-border);
background: var(--wpt-bg);
color: var(--wpt-text);
margin: 1em auto;
padding: .5em;
}
.wpt-tests-block summary {
color: var(--wptheading-text);
font-weight: normal;
text-transform: uppercase;
}
.wpt-tests-block summary::marker{
color: var(--wpt-border);
}
.wpt-tests-block summary:hover::marker{
color: var(--wpt-text);
}
/*
The only content of a wpt test block in its closed state is the <summary>,
which contains the word TESTS,
and that is absolutely positioned.
In that closed state, wpt test blocks are styled
to have a top margin whose height is exactly equal
to the height of the absolutely positioned <summary>,
and no other background/padding/margin/border.
The wpt test block elements will therefore allow the maring
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<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="https://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2021/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">CSS Containment Module Level 1</h1>
<p id="w3c-state"><a href="https://www.w3.org/standards/types#ED">Editor’s Draft</a>, <time class="dt-updated" datetime="2022-09-06">6 September 2022</time></p>
<details open>
<summary>More details about this document</summary>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://drafts.csswg.org/css-contain-1/">https://drafts.csswg.org/css-contain-1/</a>
<dt>Latest published version:
<dd><a href="https://www.w3.org/TR/css-contain-1/">https://www.w3.org/TR/css-contain-1/</a>
<dt>Previous Versions:
<dd><a href="https://www.w3.org/TR/2019/REC-css-contain-1-20191121/" rel="prev">https://www.w3.org/TR/2019/REC-css-contain-1-20191121/</a>
<dd><a href="https://www.w3.org/TR/2019/PR-css-contain-1-20191015/" rel="prev">https://www.w3.org/TR/2019/PR-css-contain-1-20191015/</a>
<dd><a href="https://www.w3.org/TR/2019/CR-css-contain-1-20190430/" rel="prev">https://www.w3.org/TR/2019/CR-css-contain-1-20190430/</a>
<dd><a href="https://www.w3.org/TR/2018/CR-css-contain-1-20181108/" rel="prev">https://www.w3.org/TR/2018/CR-css-contain-1-20181108/</a>
<dd><a href="https://www.w3.org/TR/2018/CR-css-contain-1-20180524/" rel="prev">https://www.w3.org/TR/2018/CR-css-contain-1-20180524/</a>
<dd><a href="https://www.w3.org/TR/2017/CR-css-contain-1-20170808/" rel="prev">https://www.w3.org/TR/2017/CR-css-contain-1-20170808/</a>
<dd><a href="https://www.w3.org/TR/2017/WD-css-contain-1-20170419/" rel="prev">https://www.w3.org/TR/2017/WD-css-contain-1-20170419/</a>
<dd><a href="https://www.w3.org/TR/2017/WD-css-contain-1-20170221/" rel="prev">https://www.w3.org/TR/2017/WD-css-contain-1-20170221/</a>
<dt>Implementation Report:
<dd><a href="https://drafts.csswg.org/css-contain-1/implementation-report-2022-09">https://drafts.csswg.org/css-contain-1/implementation-report-2022-09</a>
<dt>Test Suites:
<dd><a href="http://test.csswg.org/suites/css-contain-1_dev/nightly-unstable/">http://test.csswg.org/suites/css-contain-1_dev/nightly-unstable/</a>
<dd class="wpt-overview"><a href="https://wpt.fyi/results/css/css-contain/">https://wpt.fyi/results/css/css-contain/</a>
<dt>Feedback:
<dd><a href="https://github.com/w3c/csswg-drafts/labels/css-contain-1">CSSWG Issues Repository</a>
<dt class="editor">Editors:
<dd class="editor p-author h-card vcard" data-editor-id="42199"><a class="p-name fn u-url url" href="http://xanthir.com/contact/">Tab Atkins</a> (<span class="p-org org">Google</span>)
<dd class="editor p-author h-card vcard" data-editor-id="43241"><a class="p-name fn u-url url" href="https://florian.rivoal.net/">Florian Rivoal</a> (<span class="p-org org">On behalf of Bloomberg</span>)
<dt>Suggest an Edit for this Spec:
<dd><a href="https://github.com/w3c/csswg-drafts/blob/main/css-contain-1/Overview.bs">GitHub Editor</a>
</dl>
</div>
</details>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="https://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2022 <a href="https://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="https://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="https://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="https://www.keio.ac.jp/">Keio</a>, <a href="https://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="https://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document" rel="license">permissive document license</a> rules apply. </p>
<hr title="Separator for header">
</div>
<div class="p-summary" data-fill-with="abstract">
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<p>This CSS module describes the <a class="property css" data-link-type="property" href="#propdef-contain" id="ref-for-propdef-contain">contain</a> 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.</p>
<a href="https://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
</div>
<h2 class="no-num no-toc no-ref heading settled" id="sotd"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p> 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. </p>
<p>Please send feedback
by <a href="https://github.com/w3c/csswg-drafts/issues">filing issues in GitHub</a> (preferred),
including the spec code “css-contain” in the title, like this:
“[css-contain] <i>…summary of comment…</i>”.
All issues and comments are <a href="https://lists.w3.org/Archives/Public/public-css-archive/">archived</a>.
Alternately, feedback can be sent to the (<a href="https://lists.w3.org/Archives/Public/www-style/">archived</a>) public mailing list <a href="mailto:www-style@w3.org?Subject=%5Bcss-contain%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>. </p>
<p>This document is governed by the <a href="https://www.w3.org/2021/Process-20211102/" id="w3c_process_revision">2 November 2021 W3C Process Document</a>. </p>
<p></p>
<div class="correction"><a href="https://www.w3.org/Consortium/Process/#candidate-correction">Candidate corrections</a> are marked in the document.</div>
</div>
<div data-fill-with="at-risk"></div>
<nav data-fill-with="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li>
<a href="#intro"><span class="secno">1</span> <span class="content"> Introduction</span></a>
<ol class="toc">
<li><a href="#values"><span class="secno">1.1</span> <span class="content"> Value Definitions</span></a>
</ol>
<li><a href="#contain-property"><span class="secno">2</span> <span class="content"> Strong Containment: the <span class="property">contain</span> property</span></a>
<li>
<a href="#containment-types"><span class="secno">3</span> <span class="content"> Types of Containment</span></a>
<ol class="toc">
<li><a href="#containment-size"><span class="secno">3.1</span> <span class="content"> Size Containment</span></a>
<li><a href="#containment-layout"><span class="secno">3.2</span> <span class="content"> Layout Containment</span></a>
<li><a href="#containment-paint"><span class="secno">3.3</span> <span class="content"> Paint Containment</span></a>
</ol>
<li><a href="#privacy"><span class="secno">4</span> <span class="content"> Privacy Considerations</span></a>
<li><a href="#security"><span class="secno">5</span> <span class="content"> Security Considerations</span></a>
<li>
<a href="#changes"><span class="secno"></span> <span class="content">Appendix A. Changes</span></a>
<ol class="toc">
<li><a href="#2020-12-22-changes"><span class="secno"></span> <span class="content">Changes from the <span>Recommendation of 22 December 2020</span></span></a>
<li><a href="#2019-11-21-changes"><span class="secno"></span> <span class="content">Changes from the <span>Recommendation of 21 November 2019</span></span></a>
<li><a href="#2019-04-30-changes"><span class="secno"></span> <span class="content">Changes from the <span>Candidate Recommendation of 30 April 2019</span></span></a>
<li><a href="#2018-11-08-changes"><span class="secno"></span> <span class="content">Changes from the <span>Candidate Recommendation of 08 November 2018</span></span></a>
<li><a href="#2018-05-24-changes"><span class="secno"></span> <span class="content">Changes from the <span>Candidate Recommendation of 24 May 2018</span></span></a>
<li><a href="#2017-08-08-changes"><span class="secno"></span> <span class="content">Changes from the <span>Candidate Recommendation of 8 August 2017</span></span></a>
<li><a href="#2017-04-19-changes"><span class="secno"></span> <span class="content">Changes from the <span>Working Draft of 19 April 2017</span></span></a>
<li><a href="#fpwd-changes"><span class="secno"></span> <span class="content">Changes from the <span>First Public Working Draft of 21 February 2017</span></span></a>
</ol>
<li>
<a href="#w3c-conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
<ol class="toc">
<li><a href="#w3c-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
<li><a href="#w3c-conformance-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
<li>
<a href="#w3c-partial"><span class="secno"></span> <span class="content"> Partial implementations</span></a>
<ol class="toc">
<li><a href="#w3c-conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
</ol>
<li><a href="#w3c-testing"><span class="secno"></span> <span class="content"> Non-experimental implementations</span></a>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
<li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
</ol>
<li>
<a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
</ol>
<li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a>
</ol>
</nav>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content"> Introduction</span><a class="self-link" href="#intro"></a></h2>
<p>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.</p>
<p>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.</p>
<p>To alleviate these problems
and allow strong, predictable isolation of a subtree from the rest of the page,
this specification defines a <a class="property css" data-link-type="property" href="#propdef-contain" id="ref-for-propdef-contain①">contain</a> property.</p>
<h3 class="heading settled" data-level="1.1" id="values"><span class="secno">1.1. </span><span class="content"> Value Definitions</span><a class="self-link" href="#values"></a></h3>
<p>This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from <a data-link-type="biblio" href="#biblio-css2">[CSS2]</a> using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from <a data-link-type="biblio" href="#biblio-css-values-3">[CSS-VALUES-3]</a>.
Value types not defined in this specification are defined in CSS Values & Units <span>[CSS-VALUES-3]</span>.
Combination with other CSS modules may expand the definitions of these value types.</p>
<p>In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a data-link-type="dfn" href="https://drafts.csswg.org/css-values-4/#css-wide-keywords" id="ref-for-css-wide-keywords">CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.</p>
<h2 class="heading settled" data-level="2" id="contain-property"><span class="secno">2. </span><span class="content"> Strong Containment: the <a class="property css" data-link-type="property" href="#propdef-contain" id="ref-for-propdef-contain②">contain</a> property</span><a class="self-link" href="#contain-property"></a></h2>
<div class="correction" id="c1">
<span class="marker">Candidate Correction 1:</span> This makes a minor adjustment
to the way the computed value of the <a class="property css" data-link-type="property" href="#propdef-contain" id="ref-for-propdef-contain③">contain</a> property is determined:
the shortcut values (<a class="css" data-link-type="maybe" href="#valdef-contain-strict" id="ref-for-valdef-contain-strict">strict</a> and <a class="css" data-link-type="maybe" href="#valdef-contain-content" id="ref-for-valdef-contain-content">content</a>),
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 href="https://github.com/web-platform-tests/wpt/commit/98c4e2d52402156bfafcd1466985b5336f6c7240">Tests for this change</a> have 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>
</details>
</div>
<table class="def propdef" data-link-for-hint="contain">
<tbody>
<tr>
<th>Name:
<td><dfn class="dfn-paneled css" data-dfn-type="property" data-export id="propdef-contain">contain</dfn>
<tr class="value">
<th><a href="https://www.w3.org/TR/css-values/#value-defs">Value:</a>
<td class="prod">none <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-one" id="ref-for-comb-one">|</a> strict <span id="ref-for-comb-one①">|</span> content <span id="ref-for-comb-one②">|</span> [ size <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-any" id="ref-for-comb-any">||</a> layout <span id="ref-for-comb-any①">||</span> paint ]
<tr>
<th><a href="https://www.w3.org/TR/css-cascade/#initial-values">Initial:</a>
<td>none
<tr>
<th><a href="https://www.w3.org/TR/css-cascade/#applies-to">Applies to:</a>
<td>See <a href="#contain-applies">below</a>
<tr>
<th><a href="https://www.w3.org/TR/css-cascade/#inherited-property">Inherited:</a>
<td>no
<tr>
<th><a href="https://www.w3.org/TR/css-values/#percentages">Percentages:</a>
<td>n/a
<tr>
<th><a href="https://www.w3.org/TR/css-cascade/#computed">Computed value:</a>
<td>
<del cite="#c1">specified keyword(s)</del>
<ins cite="#c1">the keyword <a class="css" data-link-type="maybe" href="#valdef-contain-none" id="ref-for-valdef-contain-none">none</a> or one or more of <a class="css" data-link-type="maybe" href="#valdef-contain-size" id="ref-for-valdef-contain-size">size</a>, <a class="css" data-link-type="maybe" href="#valdef-contain-layout" id="ref-for-valdef-contain-layout">layout</a>, <a class="css" data-link-type="maybe" href="#valdef-contain-paint" id="ref-for-valdef-contain-paint">paint</a></ins>
<tr>
<th><a href="https://www.w3.org/TR/cssom/#serializing-css-values">Canonical order:</a>
<td>per grammar
<tr>
<th><a href="https://www.w3.org/TR/web-animations/#animation-type">Animation type:</a>
<td>not animatable
</table>
<details class="wpt-tests-block" dir="ltr" lang="en" open>
<summary>Tests</summary>
<ul class="wpt-tests-list">
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-animation-001.html" title="css/css-contain/contain-animation-001.html">contain-animation-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-animation-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-animation-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/inheritance.html" title="css/css-contain/inheritance.html">inheritance.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/inheritance.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/inheritance.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/parsing/contain-computed.html" title="css/css-contain/parsing/contain-computed.html">contain-computed.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/parsing/contain-computed.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/parsing/contain-computed.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/parsing/contain-invalid.html" title="css/css-contain/parsing/contain-invalid.html">contain-invalid.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/parsing/contain-invalid.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/parsing/contain-invalid.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/parsing/contain-valid.html" title="css/css-contain/parsing/contain-valid.html">contain-valid.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/parsing/contain-valid.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/parsing/contain-valid.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/parsing/contain-computed-children.html" title="css/css-contain/parsing/contain-computed-children.html">contain-computed-children.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/parsing/contain-computed-children.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/parsing/contain-computed-children.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-layout-size-003.html" title="css/css-contain/contain-layout-size-003.html">contain-layout-size-003.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-layout-size-003.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-layout-size-003.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-paint-size-001.html" title="css/css-contain/contain-paint-size-001.html">contain-paint-size-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-paint-size-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-paint-size-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-paint-size-002.html" title="css/css-contain/contain-paint-size-002.html">contain-paint-size-002.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-paint-size-002.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-paint-size-002.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-paint-size-003.html" title="css/css-contain/contain-paint-size-003.html">contain-paint-size-003.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-paint-size-003.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-paint-size-003.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-chrome-thcrash-001.html" title="css/css-contain/contain-chrome-thcrash-001.html">contain-chrome-thcrash-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-chrome-thcrash-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-chrome-thcrash-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-crash.html" title="css/css-contain/contain-crash.html">contain-crash.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-crash.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-crash.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-flexbox-outline.html" title="css/css-contain/contain-flexbox-outline.html">contain-flexbox-outline.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-flexbox-outline.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-flexbox-outline.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/crashtests/contain-nested-crash-001.html" title="css/css-contain/crashtests/contain-nested-crash-001.html">contain-nested-crash-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/crashtests/contain-nested-crash-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/crashtests/contain-nested-crash-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/crashtests/contain-nested-crash-002.html" title="css/css-contain/crashtests/contain-nested-crash-002.html">contain-nested-crash-002.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/crashtests/contain-nested-crash-002.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/crashtests/contain-nested-crash-002.html"><small>(source)</small></a>
</ul>
</details>
<p class="all-media">User agents are expected to support this property on all media, including non-visual ones.</p>
<p>The <a class="property css" data-link-type="property" href="#propdef-contain" id="ref-for-propdef-contain④">contain</a> 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 <span class="property" id="ref-for-propdef-contain⑤">contain</span> properly,
and allows authors to be confident that their page won’t accidentally fall into a slow code path
due to an innocuous change.</p>
<dl>
<dt><dfn class="dfn-paneled css" data-dfn-for="contain" data-dfn-type="value" data-export id="valdef-contain-none">none</dfn>
<dd> This value indicates that the property has no effect.
The element renders as normal,
with no containment effects applied.
<dt><dfn class="dfn-paneled css" data-dfn-for="contain" data-dfn-type="value" data-export id="valdef-contain-strict">strict</dfn>
<dd>
This value
<ins cite="#c1">computes to <span class="css">size layout paint</span>,
and thus </ins>
turns on all forms of <a data-link-type="dfn" href="#containment" id="ref-for-containment">containment</a> for the element.
<del cite="#c1">In other words, it behaves the same as <a class="css" data-link-type="propdesc" href="#propdef-contain" id="ref-for-propdef-contain⑥">contain: size layout paint;</a>.</del>
.
<details class="wpt-tests-block" dir="ltr" lang="en" open>
<summary>Tests</summary>
<ul class="wpt-tests-list">
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-strict-001.html" title="css/css-contain/contain-strict-001.html">contain-strict-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-strict-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-strict-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-strict-002.html" title="css/css-contain/contain-strict-002.html">contain-strict-002.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-strict-002.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-strict-002.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-strict-003.html" title="css/css-contain/contain-strict-003.html">contain-strict-003.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-strict-003.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-strict-003.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-flexbox-outline.html" title="css/css-contain/contain-flexbox-outline.html">contain-flexbox-outline.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-flexbox-outline.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-flexbox-outline.html"><small>(source)</small></a>
</ul>
</details>
<dt><dfn class="dfn-paneled css" data-dfn-for="contain" data-dfn-type="value" data-export id="valdef-contain-content">content</dfn>
<dd>
This value
<ins cite="#c1">computes to <span class="css">layout paint</span>,
and thus </ins>
turns on all forms of <a data-link-type="dfn" href="#containment" id="ref-for-containment①">containment</a> <em>except</em> <a data-link-type="dfn" href="#size-containment" id="ref-for-size-containment">size containment</a> for the element.
<del cite="#c1">In other words, it behaves the same as <a class="css" data-link-type="propdesc" href="#propdef-contain" id="ref-for-propdef-contain⑦">contain: layout paint;</a>.</del>
.
<details class="wpt-tests-block" dir="ltr" lang="en" open>
<summary>Tests</summary>
<ul class="wpt-tests-list">
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-content-001.html" title="css/css-contain/contain-content-001.html">contain-content-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-content-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-content-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-content-002.html" title="css/css-contain/contain-content-002.html">contain-content-002.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-content-002.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-content-002.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-content-003.html" title="css/css-contain/contain-content-003.html">contain-content-003.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-content-003.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-content-003.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-content-004.html" title="css/css-contain/contain-content-004.html">contain-content-004.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-content-004.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-content-004.html"><small>(source)</small></a>
</ul>
</details>
<p class="note" role="note"><span>Note:</span> <a class="css" data-link-type="propdesc" href="#propdef-contain" id="ref-for-propdef-contain⑧">contain: content</a> 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 data-link-type="dfn" href="#size-containment" id="ref-for-size-containment①">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 <span class="css" id="ref-for-propdef-contain⑨">contain: strict</span> when possible,
to gain as much containment as you can.</p>
<dt><dfn class="dfn-paneled css" data-dfn-for="contain" data-dfn-type="value" data-export id="valdef-contain-size">size</dfn>
<dd>
The value turns on <a data-link-type="dfn" href="#size-containment" id="ref-for-size-containment②">size containment</a> for the element.
This ensures that the <a data-link-type="dfn" href="#size-containment-box" id="ref-for-size-containment-box">containment box</a> can be laid out
without needing to examine its descendants.
<details class="wpt-tests-block" dir="ltr" lang="en" open>
<summary>Tests</summary>
<ul class="wpt-tests-list">
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-001.html" title="css/css-contain/contain-size-001.html">contain-size-001.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-001.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-001.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-002.html" title="css/css-contain/contain-size-002.html">contain-size-002.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-002.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-002.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-003.html" title="css/css-contain/contain-size-003.html">contain-size-003.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-003.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-003.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-004.html" title="css/css-contain/contain-size-004.html">contain-size-004.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-004.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-004.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-005.html" title="css/css-contain/contain-size-005.html">contain-size-005.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-005.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-005.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-006.html" title="css/css-contain/contain-size-006.html">contain-size-006.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-006.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-006.html"><small>(source)</small></a>
<li class="wpt-test"><a class="wpt-name" href="https://wpt.fyi/results/css/css-contain/contain-size-007.html" title="css/css-contain/contain-size-007.html">contain-size-007.html</a> <a class="wpt-live" href="http://wpt.live/css/css-contain/contain-size-007.html"><small>(live test)</small></a> <a class="wpt-source" href="https://github.com/web-platform-tests/wpt/blob/master/css/css-contain/contain-size-007.html"><small>(source)</small></a>
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