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<!DOCTYPE html><html lang=en><head>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Containment Module Level 3</title>
<link href=../default.css rel=stylesheet type=text/css>
<link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
:link { color: #00C; background: transparent }
:visited { color: #609; background: transparent }
a[href]:active { color: #C00; background: transparent }
a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
h1 { font: 170% sans-serif }
h2 { font: 140% sans-serif }
h3 { font: 120% sans-serif }
h4 { font: bold 100% sans-serif }
h5 { font: italic 100% sans-serif }
h6 { font: small-caps 100% sans-serif }
.hide { display: none }
div.head { margin-bottom: 1em }
div.head h1 { margin-top: 2em; clear: both }
div.head table { margin-left: 2em; margin-top: 2em }
p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
list-style: none;
}
</style>
</head>
<body class=h-entry>
<div class=head>
<p data-fill-with=logo><a class=logo href=http://www.w3.org/>
<img alt=W3C height=48 src=http://www.w3.org/Icons/w3c_home width=72>
</a>
</p>
<h1 class="p-name no-ref" id=title>CSS Containment Module Level 3</h1>
<h2 class="no-num no-toc no-ref heading settled heading" id=subtitle><span class=content>Editor’s Draft,
<span class=dt-updated><span class=value-title title=20131108>8 November 2013</span></span></span></h2>
<div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-containment/>http://dev.w3.org/csswg/css-containment/</a><dt>Editor’s Draft:<dd><a href=http://dev.w3.org/csswg/css-containment/>http://dev.w3.org/csswg/css-containment/</a>
<dt>Feedback:</dt>
<dd><a href="mailto:www-style@w3.org?subject=%5Bcss-containment%5D%20feedback">www-style@w3.org</a>
with subject line
“<kbd>[css-containment] <var>… message topic …</var></kbd>”(<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)<dt>Test Suite:<dd>None Yet<dt>Editors:
<dd class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://xanthir.com/contact/>Tab Atkins</a> (<span class="p-org org">Google</span>)</dl></div>
<div data-fill-with=warning></div>
<p class=copyright data-fill-with=copyright><a href=http://www.w3.org/Consortium/Legal/ipr-notice#Copyright>Copyright</a> © 2013 <a href=http://www.w3.org/><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href=http://www.csail.mit.edu/><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href=http://www.ercim.eu/><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href=http://www.keio.ac.jp/>Keio</a>, <a href=http://ev.buaa.edu.cn/>Beihang</a>), All Rights Reserved. W3C <a href=http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer>liability</a>, <a href=http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks>trademark</a> and <a href=http://www.w3.org/Consortium/Legal/copyright-documents>document use</a> rules apply.
</p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled heading" id=abstract><span class=content>Abstract</span></h2>
<p class=p-summary data-fill-with=abstract>This CSS module describes the <a class=property data-link-type=propdesc href=#propdef-contain title=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.
<a href=http://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, in speech, etc.</p>
<h2 class="no-num no-toc no-ref heading settled heading" id=status><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>
The (<a href=http://lists.w3.org/Archives/Public/www-style/>archived</a>) public mailing list
<a href="mailto:www-style@w3.org?Subject=%5Bcss-containment%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
(see <a href=http://www.w3.org/Mail/Request>instructions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-containment” in the subject,
preferably like this:
“[css-containment] <em>…summary of comment…</em>”
<p>
This document was produced by the <a href=http://www.w3.org/Style/CSS/members>CSS Working Group</a>
(part of the <a href=http://www.w3.org/Style/>Style Activity</a>).
<p>
This document was produced by a group operating under
the <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/>5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href=http://www.w3.org/2004/01/pp-impl/32061/status rel=disclosure>public list of any patent disclosures</a>
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential>Essential Claim(s)</a>
must disclose the information in accordance with <a href=http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure>section 6 of the W3C Patent Policy</a>.</div>
<div data-fill-with=at-risk></div>
<h2 class="no-num no-toc no-ref heading settled heading" id=contents><span class=content>Table of contents</span></h2>
<div data-fill-with=table-of-contents><ul class=toc><li><a href=#intro><span class=secno>1</span>
Introduction</a><li><a href=#containment><span class=secno>2</span>
Strong Containment: the <span class=property data-link-type=propdesc title=contain>contain</span> property</a><li><a href=#conformance><span class=secno></span>
Conformance</a><ul class=toc><li><a href=#conventions>&
8000
lt;span class=secno></span>
Document conventions</a><li><a href=#conformance-classes><span class=secno></span>
Conformance classes</a><li><a href=#partial><span class=secno></span>
Partial implementations</a><li><a href=#experimental><span class=secno></span>
Experimental implementations</a><li><a href=#testing><span class=secno></span>
Non-experimental implementations</a></ul><li><a href=#references><span class=secno></span>
References</a><ul class=toc><li><a href=#normative><span class=secno></span>
Normative References</a><li><a href=#informative><span class=secno></span>
Informative References</a></ul><li><a href=#index><span class=secno></span>
Index</a><li><a href=#property-index><span class=secno></span>
Property index</a><li><a href=#issues-index><span class=secno></span>Issues Index</a></ul></div>
<h2 class="heading settled heading" 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> 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 flunk the heuristics and fall into a slow mode.
There are also many things that would be good to isolate which are difficult or impossible to detect in a heuristic manner.
<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 data-link-type=propdesc href=#propdef-contain title=contain>contain</a> property.
<h2 class="heading settled heading" data-level=2 id=containment><span class=secno>2 </span><span class=content>
Strong Containment: the <a class=property data-link-type=propdesc href=#propdef-contain title=contain>contain</a> property</span><a class=self-link href=#containment></a></h2>
<table class="definition propdef"><tr><th>Name:<td><dfn class=css-code data-dfn-type=property data-export="" id=propdef-contain>contain<a class=self-link href=#propdef-contain></a></dfn><tr><th>Value:<td>none | strict<tr><th>Initial:<td>none<tr><th>Applies to:<td>all elements<tr><th>Inherited:<td>no<tr><th>Media:<td>all<tr><th>Computed value:<td>specified value</table>
<p> The <a class=property data-link-type=propdesc href=#propdef-contain title=contain>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 <a class=property data-link-type=propdesc href=#propdef-contain title=contain>contain</a> 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 data-dfn-for=containment data-dfn-type=value>
<dt><dfn class=css-code data-dfn-for=containment data-dfn-type=value data-export="" id=valuedef-none>none<a class=self-link href=#valuedef-none></a></dfn>
<dd>
This value indicates that the property has no effect.
The element renders as normal,
with no containment effects applied.
<dt><dfn class=css-code data-dfn-for=containment data-dfn-type=value data-export="" id=valuedef-strict>strict<a class=self-link href=#valuedef-strict></a></dfn>
<dd>
The <a class=css data-link-for=containment data-link-type=maybe href=#valuedef-strict title=strict>strict</a> value indicates that the element is <a data-link-type=dfn href=#strictly-contained title="strictly contained">strictly contained</a>,
so that its contents are guaranteed to have no effect on the rest of the page outside the element’s bounds.
</dl>
<p class=issue id=issue-1148ea1a><a class=self-link href=#issue-1148ea1a></a>
Do we want to break this apart,
so people can opt into particular forms of containment without going whole-hog,
if they really need to?
<div class=example>
<a class=property data-link-type=propdesc href=#propdef-contain title=contain>contain</a> is useful when used widely on a page,
particularly when a page contains a lot of "widgets" which are all independent.
<p> For example, assume a micropost social network had markup something like this:
<pre class=css> <body>
<aside class='sidebar'>...</aside>
<article class='messages'>
<section class='message'>
Lol, check out this dog: images.example.com/jsK3jkl
</section>
<section class='message'>
I had a ham sandwich today. #goodtimes
</section>
<section class='message'>
I have political opinions that you need to hear!
</section>
…
</article>
</body>
</pre>
<p> 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 <a class=css data-link-type=propdesc href=#propdef-contain title=contain>contain: strict</a> 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.
</div>
<p> An element that is <dfn data-dfn-type=dfn data-export="" id=strictly-contained>strictly contained<a class=self-link href=#strictly-contained></a></dfn> operates under the following restrictions:
<ol>
<li>
The contents of the element must be clipped to the element’s content box.
<li>
The element must not provide a scrolling user interface (such as a scrollbar),
nor be scrollable by any other means, such as dragging or script-based interaction.
<li>
The <a class=idl-code data-link-type=attribute title=scrollwidth>scrollWidth</a> and <a class=idl-code data-link-type=attribute title=scrollheight>scrollHeight</a> attributes of the element’s <code>DOM</code> must return the same values as the <a class=idl-code data-link-type=attribute title=clientwidth>clientWidth</a> and <a class=idl-code data-link-type=attribute title=clientheight>clientHeight</a> attributes, respectively.
<p class=issue id=issue-6081e5c9><a class=self-link href=#issue-6081e5c9></a>
Make sure this is the right way to define this. Also, ensure it’s sane.
<li>
The element must act as a containing block for absolutely positioned and fixed positioned descendants.
<li>
The element must be a <a data-link-type=dfn title="formatting context">formatting context</a>.
<li>
The following properties must have no effect on descendants of the elem
8000
ent:
<p> <ul>
<li><a class=property data-link-type=propdesc title=break-*>break-*</a> (and the related aliases)
<li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-gcpm-3/#page-group title=page-group>page-group</a>
<li><a class=property data-link-type=propdesc title=bookmark-*>bookmark-*</a>
<li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-gcpm-3/#string-set title=string-set>string-set</a>
</ul>
<li>
The <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#counter-increment title=counter-increment>counter-increment</a>, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#counter-set title=counter-set>counter-set</a>, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-regions-1/#flow-from title=flow-from>flow-from</a>, and <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-regions-1/#flow-into title=flow-into>flow-into</a> properties must be <a data-link-type=dfn href=#scoped-property title="scoped property">scoped</a> to the element’s sub-tree.
</ol>
<p> A <dfn data-dfn-type=dfn data-export="" id=scoped-property>scoped property<a class=self-link href=#scoped-property></a></dfn> has its effects scoped to a particular element or subtree.
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.
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.
<p> For example, if <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-lists-3/#counter-increment title=counter-increment>counter-increment</a> is scoped to an element,
the first use of it within the subtree acts as if the named counter were set to 0 at the scoping element,
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.
<h2 class="no-ref no-num heading settled heading" id=conformance><span class=content>
Conformance</span><a class=self-link href=#conformance></a></h2>
<h3 class="no-ref heading settled heading" id=conventions><span class=content>
Document conventions</span><a class=self-link href=#conventions></a></h3>
<p>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.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-biblio-type=normative data-link-type=biblio href=#rfc2119 title=rfc2119>[RFC2119]</a></p>
<p>Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with <code>class="example"</code>,
like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word "Note" and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.</p>
<h3 class="no-ref heading settled heading" id=conformance-classes><span class=content>
Conformance classes</span><a class=self-link href=#conformance-classes></a></h3>
<p>Conformance to this specification
is defined for three conformance classes:
<dl>
<dt>style sheet
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#style-sheet>CSS
style sheet</a>.
<dt>renderer
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
that interprets the semantics of a style sheet and renders
documents that use them.
<dt>authoring tool
<dd>A <a href=http://www.w3.org/TR/CSS21/conform.html#user-agent>UA</a>
that writes a style sheet.
</dl>
<p>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.
<p>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.)
<p>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.
<h3 class="no-ref heading settled heading" id=partial><span class=content>
Partial implementations</span><a class=self-link href=#partial></a></h3>
<p>So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers <strong>must</strong>
treat as invalid (and <a href=http://www.w3.org/TR/CSS21/conform.html#ignore>ignore
as appropriate</a>) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents <strong>must not</strong> 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.</p>
<h3 class="no-ref heading settled heading" id=experimental><span class=content>
Experimental implementations</span><a class=self-link href=#experimental></a></h3>
<p>To avoid clashes with future CSS features, the CSS2.1 specification
reserves a <a href=http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords>prefixed
syntax</a> for proprietary and experimental extensions to CSS.
<p>Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.
</p>
<h3 class="no-ref heading settled heading" id=testing><span class=content>
Non-experimental implementations</span><a class=self-link href=#testing></a></h3>
<p>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.
<p>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.
<p>Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group’s website at
<a href=http://www.w3.org/Style/CSS/Test/>http://www.w3.org/Style/CSS/Test/</a>.
Questions should be directed to the
<a href=http://lists.w3.org/Archives/Public/public-css-testsuite>public-css-testsuite@w3.org</a>
mailing list.
<h2 class="no-num no-ref heading settled heading" id=references><span class=content>
References</span><a class=self-link href=#references></a></h2>
<h3 class="no-num no-ref heading settled heading" id=normative><span class=content>
Normative References</span><a class=self-link href=#normative></a></h3>
<div data-fill-with=normative-references><dl><dt id=rfc2119 title=RFC2119><a class=self-link href=#rfc2119></a>[RFC2119]<dd>S. Bradner. <a href=http://www.ietf.org/rfc/rfc2119.txt>Key words for use in RFCs to Indicate Requirement Levels</a>. URL: <a href=http://www.ietf.org/rfc/rfc2119.txt>http://www.ietf.org/rfc/rfc2119.txt</a></dl></div>
<h3 class="no-num no-ref heading settled heading" id=informative><span class=content>
Informative References</span><a class=self-link href=#informative></a></h3>
<div data-fill-with=informative-references><dl></dl></div>
<h2 class="no-num no-ref heading settled heading" id=index><span class=content>
Index</span><a class=self-link href=#index></a></h2>
<div data-fill-with=index><ul class=indexlist>
<li>contain, <a href=#propdef-contain title="section 2">2</a>
<li>none, <a href=#valuedef-none title="section 2">2</a>
<li>scoped property, <a href=#scoped-property title="section 2">2</a>
<li>strict, <a href=#valuedef-strict title="section 2">2</a>
<li>strictly contained, <a href=#strictly-contained title="section 2">2</a>
</ul></div>
<h2 class="no-num no-ref heading settled heading" id=property-index><span class=content>
Property index</span><a class=self-link href=#property-index></a></h2>
<div data-fill-with=property-index><table class="proptable data"><thead><tr><th scope=col>Name<th scope=col>Value<th scope=col>Initial<th scope=col>Applies to<th scope=col>Inh.<th scope=col>%ages<th scope=col>Media<th scope=col>Computed value<tbody>
<tr><th scope=row><a data-property="">contain</a><td>none | strict<td>none<td>all elements<td>no<td><td>all<td>specified value</table></div>
<h2 class="no-num heading settled" id=issues-index><span class=content>Issues Index</span><a class=self-link href=#issues-index></a></h2><div style="counter-reset: issue"><div class=issue>
Do we want to break this apart,
so people can opt into particular forms of containment without going whole-hog,
if they really need to?
<a href=#issue-1148ea1a> ↵ </a></div><div class=issue>
Make sure this is the right way to define this. Also, ensure it’s sane.
<a href=#issue-6081e5c9> ↵ </a></div></div>