csswg-drafts/css-scoping/Overview.html at d3366dc095f658d150700cbb64b1d447aaf747d4 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html><html lang=en><head>
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Scoping Module Level 1</title>
  <link href=../default.css rel=stylesheet type=text/css>
  <link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
  <style>
  body {
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  a[href] img { border-style: none }

  h1, h2, h3, h4, h5, h6 { text-align: left }
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  div.head table { margin-left: 2em; margin-top: 2em }

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  pre { margin-left: 2em }
  dt { font-weight: bold }

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    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=https://www.w3.org/Icons/w3c_home width=72>
</a>
</p>
  <h1 class="p-name no-ref" id=title>CSS Scoping Module Level 1</h1>
  <h2 class="no-num no-toc no-ref heading settled" id=subtitle><span class=content>Editor’s Draft,
    <span class=dt-updated><span class=value-title title=20141121>21 November 2014</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-scoping/>http://dev.w3.org/csswg/css-scoping/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css-scoping-1/>http://www.w3.org/TR/css-scoping-1/</a><dt>Feedback:<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-scoping%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-scoping] <var>… message topic …</var></kbd>” (<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)</span><dt>Issue Tracking:<dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?component=Scoping&amp;list_id=47685&amp;product=CSS&amp;resolution=---">Bugzilla</a><dd><a href=#issues-index>Inline In Spec</a><dt class=editor>Editors:<dd class=editor><div class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://xanthir.com/contact/>Tab Atkins Jr.</a> (<span class="p-org org">Google</span>)</div><dd class=editor><div class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://fantasai.inkedblade.net/contact>Elika J Etemad</a> (<span class="p-org org">Invited Expert</span>)</div></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> © 2014 <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" id=abstract><span class=content>Abstract</span></h2>
<div class=p-summary data-fill-with=abstract><p>This specification defines various scoping/encapsulation mechanisms for CSS, including scoped styles and the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rule, Shadow DOM selectors, and page/region-based styling.</p>

<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.</div>

<h2 class="no-num no-toc no-ref heading settled" 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-scoping%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-scoping” in the subject,
	preferably like this:
	“[css-scoping] <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>.

<p>
	This document is governed by the <a href=http://www.w3.org/2014/Process-20140801/>1 August 2014 W3C Process Document</a>.
</div>
<div data-fill-with=at-risk></div>

<h2 class="no-num no-toc no-ref heading settled" id=contents><span class=content>Table of Contents</span></h2>
<div data-fill-with=table-of-contents role=navigation><ul class=toc role=directory><li><a href=#intro><span class=secno>1</span> <span class=content>
Introduction</span></a><li><a href=#scope><span class=secno>2</span> <span class=content>
Scoped Styles</span></a><ul class=toc><li><a href=#scoping-mechanisms><span class=secno>2.1</span> <span class=content>
Scoping Mechanisms</span></a><ul class=toc><li><a href=#scoping-markup><span class=secno>2.1.1</span> <span class=content>
Document Markup for Scoping</span></a><li><a href=#scope-atrule><span class=secno>2.1.2</span> <span class=content>
CSS Syntax for Scoping: the <span class=css data-link-type=maybe title=@scope>@scope</span> rule</span></a></ul><li><a href=#scoping-context><span class=secno>2.2</span> <span class=content>
Querying the Scoping Context</span></a><ul class=toc><li><a href=#scope-pseudo><span class=secno>2.2.1</span> <span class=content>
Selecting the Scoping Root: <span class=css data-link-type=maybe title=:scope>:scope</span> pseudo-class</span></a><li><a href=#scope-content-pseudo><span class=secno>2.2.2</span> <span class=content>
Selecting Outside the Scope: <span class=css>:scope-context()</span> pseudo-class</span></a></ul></ul><li><a href=#shadow-dom><span class=secno>3</span> <span class=content>
Shadow Encapsulation</span></a><ul class=toc><li><a href=#selectors-data-model><span class=secno>3.1</span> <span class=content>
Shadow DOM Selection Model</span></a><ul class=toc><li><a href=#host-element-in-tree><span class=secno>3.1.1</span> <span class=content>
Host Elements in a Shadow Tree</span></a></ul><li><a href=#selectors><span class=secno>3.2</span> <span class=content>
Shadow DOM Selectors</span></a><ul class=toc><li><a href=#host-selector><span class=secno>3.2.1</span> <span class=content>
Selecting Into the Light: the <span class=css data-link-type=maybe title=:host>:host</span>, <span class=css data-link-type=maybe title=:host()>:host()</span>, and <span class=css data-link-type=maybe title=:host-context()>:host-context()</span> pseudo-classes</span></a><li><a href=#shadow-pseudoelement><span class=secno>3.2.2</span> <span class=content>
Selecting Into the Dark: the <span class=css data-link-type=maybe title=::shadow>::shadow</span> pseudo-element</span></a><li><a href=#content-combinator><span class=secno>3.2.3</span> <span class=content>
Selecting Shadow-Projected Content: the <span class=css data-link-type=maybe title=::content>::content</span> pseudo-element</span></a><li><a href=#deep-combinator><span class=secno>3.2.4</span> <span class=content>
Selecting Through Shadows: the <span class=css data-link-for="">deep/</span> combinator</span></a></ul><li><a href=#shadow-cascading><span class=secno>3.3</span> <span class=content>
Shadow Cascading &amp; Inheritance</span></a><ul class=toc><li><a href=#cascading><span class=secno>3.3.1</span> <span class=content>
Cascading</span></a><li><a href=#inheritance><span class=secno>3.3.2</span> <span class=content>
Inheritance</span></a></ul></ul><li><a href=#fragment-scoping><span class=secno>4</span> <span class=content>
Fragmented Styling</span></a><ul class=toc><li><a href=#the-region-pseudo-element><span class=secno>4.1</span> <span class=content>
Region-based Styling: the <span class=css>::region</span> pseudo-element</span></a></ul><li><a href=#conformance><span class=secno></span> <span class=content>
Conformance</span></a><ul class=toc><li><a href=#conventions><span class=secno></span> <span class=content>
Document conventions</span></a><li><a href=#conformance-classes><span class=secno></span> <span class=content>
Conformance classes</span></a><li><a href=#partial><span class=secno></span> <span class=content>
Partial implementations</span></a><li><a href=#experimental><span class=secno></span> <span class=content>
Experimental implementations</span></a><li><a href=#testing><span class=secno></span> <span class=content>
Non-experimental implementations</span></a></ul><li><a href=#references><span class=secno></span> <span class=content>References</span></a><ul 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></ul><li><a href=#index><span class=secno></span> <span class=content>Index</span></a><li><a href=#issues-index><span class=secno></span> <span class=content>Issues Index</span></a></ul></div>
<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>...</p>


<h2 class="heading settled" data-level=2 id=scope><span class=secno>2. </span><span class=content>
Scoped Styles</span><a class=self-link href=#scope></a></h2>

<p><a data-link-type=dfn href=http://dev.w3.org/csswg/css-cascade-3/#scoped title=Scoped>Scoped</a> style rules apply only within a subtree of a document,
	rather than matching against the entire document.
	Scoping has two primary effects:</p>
	<ul>
		<li>The selector of the <a data-link-type=dfn href=http://dev.w3.org/csswg/css-cascade-3/#scoped title=scoped>scoped</a> style rule is restricted to match only elements within scope.
		See <a href=http://www.w3.org/TR/selectors4/#scoping>Scoped Selectors</a> in <a data-biblio-type=informative data-link-type=biblio href=#biblio-selectors4 title=SELECTORS4>[SELECTORS4]</a>.
		<li>The cascade prioritizes scoped rules over unscoped ones, regardless of specificity.
		See <a href=http://www.w3.org/TR/css-cascade/#cascade-scope>Cascading by Scope</a> in <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3cascade title=CSS3CASCADE>[CSS3CASCADE]</a>.
	</ul>

<h3 class="heading settled" data-level=2.1 id=scoping-mechanisms><span class=secno>2.1. </span><span class=content>
Scoping Mechanisms</span><a class=self-link href=#scoping-mechanisms></a></h3>

<p>Style rules can be scoped using constructs defined in the document language
	or using the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rule in CSS.</p>

<p>“Scoping” consists of three somewhat independent concepts,
	which are in practice generally used together:</p>

	<ul>
		<li>
			A declaration can be scoped to <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>,
			which <a href=http://dev.w3.org/csswg/css-cascade/#cascade-scope>affects its cascading behavior</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-css3cascade title=CSS3CASCADE>[CSS3CASCADE]</a>
			(Alternately, a style rule can be scoped to <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>,
			which scopes all of the declarations it contains to that <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>.)

		<li>
			A selector can be either <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-contained title=scope-contained>scope-contained</a> or <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-filtered title=scope-filtered>scope-filtered</a> to a <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>,
			which <a href=http://dev.w3.org/csswg/selectors/#evaluating-selectors>limits what elements it is allowed to match</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-selectors4 title=SELECTORS4>[SELECTORS4]</a>

		<li>
			The <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/selectors-4/#scope-pseudo title=:scope>:scope</a> pseudo-class matches whatever the context sets as the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-element title=":scope elements">:scope elements</a>,
			and is used by several features, such as relative selectors.
			In the absence of any anything explicitly setting the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-element title=":scope elements">:scope elements</a> to something,
			the <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/selectors-4/#scope-pseudo title=:scope>:scope</a> pseudo-class matches the selector’s <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>.
	</ul>

<h4 class="heading settled" data-level=2.1.1 id=scoping-markup><span class=secno>2.1.1. </span><span class=content>
Document Markup for Scoping</span><a class=self-link href=#scoping-markup></a></h4>

<p>Document languages may define a mechanism for a stylesheet to be scoped to some element in the document.
	For example, in HTML,
	a <a data-link-type=element href=https://html.spec.whatwg.org/#the-style-element title=style>style</a> element with a <a data-link-for=style data-link-type=element-attr title=scoped>scoped</a> attribute
	defines a stylesheet that is scoped to the <a data-link-type=element href=https://html.spec.whatwg.org/#the-style-element title=style>style</a> element’s parent element.
	<a data-biblio-type=informative data-link-type=biblio href=#biblio-html title=HTML>[HTML]</a></p>

<p>The element that the stylesheet is scoped to
	is the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a> for all the style rules in the stylesheet,
	and selectors of style rules in the stylesheet
	are <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-contained title=scope-contained>scope-contained</a> to the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>.</p>

<h4 class="heading settled" data-level=2.1.2 id=scope-atrule><span class=secno>2.1.2. </span><span class=content>
CSS Syntax for Scoping: the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rule</span><a class=self-link href=#scope-atrule></a></h4>

<p>The <dfn class=css data-dfn-type=at-rule data-export="" id=at-ruledef-scope>@scope<a class=self-link href=#at-ruledef-scope></a></dfn> at-rule allows authors to create scoped style rules using CSS syntax.
	The syntax of the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rule is:</p>

	<pre class=prod>@scope <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a> {
  <a class=production data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-stylesheet title="<stylesheet>">&lt;stylesheet&gt;</a>
}
</pre>

<p>where the elements matched by the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a>
	are <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping roots">scoping roots</a> for the style rules in <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-stylesheet title="<style
6111
sheet>">&lt;stylesheet&gt;</a>,
	and selectors of style rules scoped by <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> are
	<a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scope-contained title=scope-contained>scope-contained</a> to their <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>.</p>

	<p class=issue id=issue-5f568f10><a class=self-link href=#issue-5f568f10></a>
	This rule makes it very easy for authors to create scoped style sheets,
	which could affect the optimization strategies for implementing scoped styles.

<p>If multiple elements match the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a>,
	the <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/css-syntax-3/#typedef-stylesheet title="<stylesheet>">&lt;stylesheet&gt;</a> is effectively duplicated
	and scoped independently to each one.
	Authors should avoid using overly-generic selectors
	as it can have confusing interactions with the cascade.</p>

	<div class=example>
		A scoped stylesheet is attached not only to the outermost scoping element,
		but to all matching elements.
		For example, given the style sheet below
		<pre>@scope div {
  span {
    color: blue;
  }
}
@scope section {
  span {
    color: orange;
  }
}
</pre>

<p>and the following document fragment</p>

		<pre>&lt;div&gt;
  &lt;section&gt;
    &lt;div&gt;
      &lt;span&gt;text&lt;/span&gt;
    &lt;/div&gt;
  &lt;/section&gt;
&lt;/div&gt;
</pre>

<p>the text will be blue.</p>
	</div>

<p><a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rules can be nested.
	In this case, just as with the nested style rules,
	the selector of an outer <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> scope-contains
	the selector of the inner one.</p>

<p>The specificity of selectors inside the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rule is calculated locally:
	the selector specifying the scoping element is ignored.
	However, because scoped styles override non-scoped styles,
	style rules inside the <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> will override rules outside of it.</p>

	<div class=example>
		In the following example, the text would be green:

		<pre>@scope aside {
  p { color: green; }
}
aside#sidebar p { color: red; }
</pre>
	</div>

	<p class=issue id=issue-f07dce09><a class=self-link href=#issue-f07dce09></a>
	If multiple <a class=css data-link-type=maybe href=#at-ruledef-scope title=@scope>@scope</a> rules apply to an element,
	should they be cascaded by specificity?

<h3 class="heading settled" data-level=2.2 id=scoping-context><span class=secno>2.2. </span><span class=content>
Querying the Scoping Context</span><a class=self-link href=#scoping-context></a></h3>

<h4 class="heading settled" data-level=2.2.1 id=scope-pseudo><span class=secno>2.2.1. </span><span class=content>
Selecting the Scoping Root: <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/selectors-4/#scope-pseudo title=:scope>:scope</a> pseudo-class</span><a class=self-link href=#scope-pseudo></a></h4>

<p>In a scoped stylesheet,
	the <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/selectors-4/#scope-pseudo title=:scope>:scope</a> pseudo-class,
	defined in <a data-biblio-type=informative data-link-type=biblio href=#biblio-selectors4 title=SELECTORS4>[SELECTORS4]</a>,
	matches the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a>.</p>

<h4 class="heading settled" data-level=2.2.2 id=scope-content-pseudo><span class=secno>2.2.2. </span><span class=content>
Selecting Outside the Scope: <span class=css>:scope-context()</span> pseudo-class</span><a class=self-link href=#scope-content-pseudo></a></h4>

	<div class=issue id=issue-6bd6b9cc><a class=self-link href=#issue-6bd6b9cc></a>
		This would be defined similarly to <a class=css data-link-type=maybe href=#selectordef-host-context title=:host-context()>:host-context()</a>,
		but matching the ancestors of the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#scoping-root title="scoping root">scoping root</a> instead.

<p>However, since for scoped stylesheets you may want the ability to match complex selectors against the outside tree,
		rather than a single compound selector,
		we may want to instead use a more general mechanism that doesn’t syntactically invert the order of tree elements.</p>

<p>Possible ideas:</p>

		<pre>:scope-context(<a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a>) div {...}
scope(<a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a>) div {...}
\scope <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a>\ div {...}
<a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-selector title="<selector>">&lt;selector&gt;</a> \scope\ div {...}
</pre>

<p>This functionality would replace <span class=css>@global</span>, which is a poor excuse for a selector.</p>
	</div>

<h2 class="heading settled" data-level=3 id=shadow-dom><span class=secno>3. </span><span class=content>
Shadow Encapsulation</span><a class=self-link href=#shadow-dom></a></h2>

<p>The Shadow DOM spec augments the DOM with several new concepts,
	several of which are relevant to CSS.</p>

<p>A <dfn data-dfn-type=dfn data-export="" id=shadow-tree>shadow tree<a class=self-link href=#shadow-tree></a></dfn> is a document fragment
	that can be attached to any element in the DOM.
	The root of the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> is a <dfn data-dfn-type=dfn data-export="" id=shadow-root>shadow root<a class=self-link href=#shadow-root></a></dfn>,
	a non-element node which is associated with a <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a>.
	An element can have any number of <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>,
	which are ordered by creation time.
	The most recently-created <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> on an element
	is the <dfn data-dfn-type=dfn data-export="" id=active-shadow-tree>active shadow tree<a class=self-link href=#active-shadow-tree></a></dfn> for that element.</p>

<p>An element with a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> is a <dfn data-dfn-type=dfn data-export="" id=shadow-host>shadow host<a class=self-link href=#shadow-host></a></dfn>.
	It is the <dfn data-dfn-type=dfn data-export="" id=host-element>host element<a class=self-link href=#host-element></a></dfn> for its shadow trees.</p>

<p>The descendants of a <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a>
	must not generate boxes in the formatting tree.
	Instead, the contents of the <a data-link-type=dfn href=#active-shadow-tree title="active shadow tree">active shadow tree</a> generate boxes
	as if they were the contents of the element instead.</p>

<p>In several instances in shadow DOM,
	elements don’t have element parents
	(instead, they may have a <a data-link-type=dfn href=#shadow-root title="shadow root">shadow root</a> as parent,
		or something else).
	An element without a parent,
	or whose parent is not an element,
	is called a <dfn data-dfn-type=dfn data-export="" id=top-level-element>top-level element<a class=self-link href=#top-level-element></a></dfn>.</p>

<p>While the children of a <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a> do not generate boxes normally,
	they can be explicitly pulled into a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> and forced to render normally.
	This is done by assigning the elements to a <dfn data-dfn-type=dfn data-export="" id=distribution-list>distribution list<a class=self-link href=#distribution-list></a></dfn>.
	An element with a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a> is an <dfn data-dfn-type=dfn data-export="" id=insertion-point>insertion point<a class=self-link href=#insertion-point></a></dfn>.</p>

<p>This specification does not define how to assign elements to a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a>,
	instead leaving that to the Shadow DOM spec.
	At the time this spec is written, however,
	only <a data-link-type=element title=content>content</a> elements in a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> can have <a data-link-type=dfn href=#distribution-list title="distribution lists">distribution lists</a>.</p>

<p>An <a data-link-type=dfn href=#insertion-point title="insertion point">insertion point</a> must not generate any boxes.
	Instead, the elements in its <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a> generate boxes as normal,
	as if they all replaced the <a data-link-type=dfn href=#insertion-point title="insertion point">insertion point</a> in-place.
	<span class=note role=note>(Akin to the behavior of <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css-display-3/#propdef-display-outside title=display-outside>display-outside: contents</a>.)</span></p>

<h3 class="heading settled" data-level=3.1 id=selectors-data-model><span class=secno>3.1. </span><span class=content>
Shadow DOM Selection Model</span><a class=self-link href=#selectors-data-model></a></h3>

<p>Elements in the DOM
	have zero or more <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>
	and zero or one <a data-link-type=dfn href=#distribution-list title="distribution lists">distribution lists</a>.</p>

<p class=note role=note>Note: The "descendants" of an element
	are based on the children of the element,
	which does not include the <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a> or <a data-link-type=dfn href=#distribution-list title="distribution lists">distribution lists</a> of the element.</p>

<p>When a selector is matched against a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#selector-match-list title="selector match list">selector match list</a>
	is initially the <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a>,
	followed by all the <a data-link-type=dfn href=#top-level-element title="top-level elements">top-level elements</a> of the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>
	and their descendants,
	ordered by a pre-order traversal.</p>

<h4 class="heading settled" data-level=3.1.1 id=host-element-in-tree><span class=secno>3.1.1. </span><span class=content>
Host Elements in a Shadow Tree</span><a class=self-link href=#host-element-in-tree></a></h4>

<p>A <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a> is outside of the <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a> it hosts,
	but it is sometimes useful to be able to style it from inside the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> context.</p>

<p>For the purpose of Selectors,
	a <a data-link-type=dfn href=#host-element title="host element">host element</a> also appears in each of its <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>,
	with the contents of the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> treated as its children.
	If an element has multiple <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>,
	it appears in each <a data-link-type=dfn href=#shadow-tree title="shadow tree’s">shadow tree’s</a> context independently;
	each <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> sees <em>itself</em> as the contents of the <a data-link-type=dfn href=#host-element title="host element">host element</a>,
	not the other <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>.</p>

<p>The <a data-link-type=dfn href=#host-element title="host element">host element</a> is not selectable by <strong>any means</strong>
	except for the <a class=css data-link-type=maybe href=#selectordef-host title=:host>:host</a> and <a class=css data-link-type=maybe href=#selectordef-host-context title=:host-context()>:host-context()</a> pseudo-classes.
	That is, in this context the <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a>
	has no tagname, ID, classes, or attributes,
	and the only additional information is has is that the <a class=css data-link-type=maybe href=#selectordef-host title=:host>:host</a> pseudo-class matches it.
	In particular, the <a data-link-type=dfn href=#host-element title="host element">host element</a> isn’t matched by the <span class=css>*</span> selector either.</p>

	<details class=why>
		<summary>Why is the shadow host so weird?</summary>

<p>The <a data-link-type=dfn href=#shadow-host title="shadow host">shadow host</a> lives outside the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
		and its markup is in control of the page author,
		not the component author.</p>

<p>It would not be very good if a component used a particular class name
		internally in a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
		and the page author using the component accidentally <em>also</em>
		used the the same class name and put it on the <a data-link-type=dfn href=#host-element title="host element">host element</a>.
		Such a situation would result in accidental styling
		that is impossible for the component author to predict,
		and confusing for the page author to debug.</p>

<p>However, there are still some reasonable use-cases for letting a stylesheet in a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>
		style its <a data-link-type=dfn href=#host-element title="host element">host element</a>.
		So, to allow this situation but prevent accidental styling,
		the <a data-link-type=dfn href=#host-element title="host element">host element</a> appears but is completely featureless
		and unselectable except through <a class=css data-link-type=maybe href=#selectordef-host title=:host>:host</a>.</p>
	</details>


<h3 class="heading settled" data-level=3.2 id=selectors><span class=secno>3.2. </span><span class=content>
Shadow DOM Selectors</span><a class=self-link href=#selectors></a></h3>

<p>Shadow DOM defines a few new selectors
	to help select elements in useful way related to Shadow DOM.</p>

	<p class=issue id=issue-dea392ae><a class=self-link href=#issue-dea392ae></a>
	This section is still under discussion.
	Feedback and advice on intuitive syntax for the following functionality
	would be appreciated.

<h4 class="heading settled" data-level=3.2.1 id=host-selector><span class=secno>3.2.1. </span><span class=content>
Selecting Into the Light: the <a class=css data-link-type=maybe href=#selectordef-host title=:host>:host</a>, <a class=css data-link-type=maybe href=#selectordef-host-function title=:host()>:host()</a>, and <a class=css data-link-type=maybe href=#selectordef-host-context title=:host-context()>:host-context()</a> pseudo-classes</span><a class=self-link href=#host-selector></a></h4>

<p>The <dfn class=css data-dfn-type=selector data-export="" id=selectordef-host>:host<a class=self-link href=#selectordef-host&g
6111
t;</a></dfn> pseudo-class,
	when evaluated in the context of a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	matches the <a data-link-type=dfn href=#shadow-tree title="shadow tree’s">shadow tree’s</a> <a data-link-type=dfn href=#host-element title="host element">host element</a>.
	In any other context,
	it matches nothing.</p>

<p>The <dfn class=css data-dfn-type=selector data-export="" id=selectordef-host-function>:host()<a class=self-link href=#selectordef-host-function></a></dfn> function pseudo-class
	has the syntax:</p>

<pre>:host( <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-compound-selector title="<compound-selector>">&lt;compound-selector&gt;</a> )</pre>

<p>When evaluated in the context of a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	it matches the <a data-link-type=dfn href=#shadow-tree title="shadow tree’s">shadow tree’s</a> <a data-link-type=dfn href=#host-element title="host element">host element</a>
	if the <a data-link-type=dfn href=#host-element title="host element">host element</a>,
	in its normal context,
	matches the selector argument.
	In any other context,
	it matches nothing.</p>

	<div class=example>
		For example, say you had a component with a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> like the following:

		<pre>&lt;x-foo class="foo"&gt;
  &lt;"shadow tree"&gt;
    &lt;div class="foo"&gt;...&lt;/div&gt;
  &lt;/&gt;
&lt;/x-foo&gt;
</pre>

<p>For a stylesheet within the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>:</p>

		<ul>
			<li><a class=css data-link-type=maybe href=#selectordef-host title=:host>:host</a> matches the <code>&lt;x-foo&gt;</code> element.
			<li><span class=css>x-foo</span> matches nothing.
			<li><span class=css>.foo</span> matches only the <code>&lt;div&gt;</code> element.
			<li><span class=css>.foo:host</span> matches nothing
			<li><span class=css>:host(.foo)</span> matches the <code>&lt;x-foo&gt;</code> element.
		</ul>
	</div>

<p>Ordinary, selectors within a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>
	can’t see elements outside the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> at all.
	Sometimes, however, it’s useful to select an ancestor that lies somewhere outside the shadow tree,
	above it in the document.</p>

	<div class=example>
		For example, a group of components can define a handful of color themes
		they they know how to respond to.
		Page authors could opt into a particular theme
		by adding a specific class to the components,
		or higher up in the document.
	</div>

<p>The <dfn class=css data-dfn-type=selector data-export="" id=selectordef-host-context>:host-context()<a class=self-link href=#selectordef-host-context></a></dfn> functional pseudo-class tests whether there is an ancestor,
	outside the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	which matches a particular selector.
	Its syntax is:</p>

<pre>:host-context( <a class=production data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-compound-selector title="<compound-selector>">&lt;compound-selector&gt;</a> )</pre>

<p>When evaluated in the context of a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	the <a class=css data-link-type=maybe href=#selectordef-host-context title=:host-context()>:host-context()</a> pseudo-class matches the <a data-link-type=dfn href=#host-element title="host element">host element</a>,
	if the <a data-link-type=dfn href=#host-element title="host element">host element</a> or one of its ancestors matches the provided <a class="production css" data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#typedef-compound-selector title="<compound-selector>">&lt;compound-selector&gt;</a>.
	For the purpose of this pseudo-class,
	the "ancestor" of an element is:</p>

	<dl>
		<dt>if the element is distributed to a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a>
		<dd>
			the <a data-link-type=element title=content>content</a> element it is ultimately distributed to.

		<dt>if the element is a top-most element in a shadow tree
		<dd>
			the <a data-link-type=dfn href=#host-element title="host element">host element</a>

		<dt>otherwise
		<dd>
			the element’s parent,
			if it has one.
	</dl>

<p class=note role=note>Note: This means that the selector pierces through shadow boundaries on the way up,
	looking for elements that match its argument,
	until it reaches the document root.</p>

<h4 class="heading settled" data-level=3.2.2 id=shadow-pseudoelement><span class=secno>3.2.2. </span><span class=content>
Selecting Into the Dark: the <a class=css data-link-type=maybe href=#selectordef-shadow title=::shadow>::shadow</a> pseudo-element</span><a class=self-link href=#shadow-pseudoelement></a></h4>

<p>If an element has at least one <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	the <dfn class=css data-dfn-type=selector data-export="" id=selectordef-shadow>::shadow<a class=self-link href=#selectordef-shadow></a></dfn> pseudo-element matches the <a data-link-type=dfn href=#shadow-root title="shadow roots">shadow roots</a> themselves.
	In HTML, the <a data-link-type=dfn href=#shadow-root title="shadow root">shadow root</a> is represented by <a class=idl-code data-link-type=interface title=ShadowRoot>ShadowRoot</a> objects.</p>

<p>The <a class=css data-link-type=maybe href=#selectordef-shadow title=::shadow>::shadow</a> pseudo-element must not generate boxes,
	unless specified otherwise in another specification.
	However, for the purpose of Selectors,
	the <a class=css data-link-type=maybe href=#selectordef-shadow title=::shadow>::shadow</a> pseudo-element is considered to be the root of the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
	with the <a data-link-type=dfn href=#top-level-element title="top-level elements">top-level elements</a> in the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> the direct children of the <a class=css data-link-type=maybe href=#selectordef-shadow title=::shadow>::shadow</a> pseudo-element.</p>

	<div class=example>
		For example, say you had a component with a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> like the following:

		<pre>&lt;x-foo&gt;
  &lt;"shadow tree"&gt;
    &lt;div&gt;
      &lt;span id="not-top"&gt;...&lt;/span&gt;
    &lt;/div&gt;
    &lt;span id="top"&gt;...&lt;/span&gt;
  &lt;/&gt;
&lt;/x-foo&gt;
</pre>

<p>For a stylesheet in the outer document,
		<span class=css>x-foo::shadow &gt; span</span> matches <span class=css>#top</span>,
		but not <span class=css>#not-top</span>,
		because it’s not a <a data-link-type=dfn href=#top-level-element title="top-level element">top-level element</a> in the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>.</p>

<p>If one wanted to target <span class=css>#not-top</span>,
		one way to do it would be with <span class=css>x-foo::shadow &gt; div &gt; span</span>.
		However, this introduces a strong dependency on the internal structure of the component;
		in most cases, it’s better to use the descendant combinator,
		like <span class=css>x-foo::shadow span</span>,
		to select all the elements of some type in the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>.</p>
	</div>

	<div class=example>
		If an element has multiple <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a>,
		a <a class=css data-link-type=maybe href=#selectordef-shadow title=::shadow>::shadow</a> pseudo-element selects <em>all</em> of the corresponding <a data-link-type=dfn href=#shadow-root title="shadow roots">shadow roots</a>.

<p>Similarly,
		inside of a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
		a selector like <span class=css>:host::shadow div</span> selects the <a data-link-type=element href=https://html.spec.whatwg.org/#the-div-element title=div>div</a> elements in <em>all</em> the <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a> on the element,
		not just the one containing that selector.</p>
	</div>


<h4 class="heading settled" data-level=3.2.3 id=content-combinator><span class=secno>3.2.3. </span><span class=content>
Selecting Shadow-Projected Content: the <a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> pseudo-element</span><a class=self-link href=#content-combinator></a></h4>

<p>The <dfn class=css data-dfn-type=selector data-export="" id=selectordef-content>::content<a class=self-link href=#selectordef-content></a></dfn> pseudo-element matches the <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a> itself,
	on elements that have one.</p>

	<p class=issue id=issue-a514442b><a class=self-link href=#issue-a514442b></a>
	<a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> is a confusingly general name for something that is specific
	to the projected content of a shadow tree.

<p>The <a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> pseudo-element must not generate boxes,
	unless specified otherwise in another specification.
	However, for the purpose of Selectors,
	the <a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> pseudo-element is considered to be the parent of the elements in the <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a>.</p>

	<div class=example>
		For example, say you had a component with both children and a shadow tree,
		like the following:

		<pre>&lt;x-foo&gt;
  &lt;div id="one" class="foo"&gt;...&lt;/div&gt;
  &lt;div id="two"&gt;...&lt;/div&gt;
  &lt;div id="three" class="foo"&gt;
    &lt;div id="four"&gt;...&lt;/div&gt;
  &lt;/div&gt;
  &lt;"shadow tree"&gt;
    &lt;div id="five"&gt;...&lt;/div&gt;
    &lt;div id="six"&gt;...&lt;/div&gt;
    &lt;content select=".foo"&gt;&lt;/content&gt;
  &lt;/"shadow tree"&gt;
&lt;/x-foo&gt;
</pre>

<p>For a stylesheet within the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
		a selector like <span class=css>::content div</span>
		selects <span class=css>#one</span>, <span class=css>#three</span>, and <span class=css>#four</span>,
		as they’re the elements distributed by the sole <a data-link-type=element title=content>content</a> element,
		but not <span class=css>#two</span>.</p>

<p>If only the <a data-link-type=dfn href=#top-level-element title="top-level elements">top-level elements</a> distributed the <a data-link-type=element title=content>content</a> element are desired,
		a <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#child-combinator title="child combinator">child combinator</a> can be used,
		like <span class=css>::content &gt; div</span>,
		which will exclude <span class=css>#four</span>
		as it’s not treated as a child of the <a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> pseudo-element.</p>

<p class=note role=note>Note: Note that a selector like <span class=css>::content div</span>
		is equivalent to <span class=css>*::content div</span>,
		where the <span class=css>*</span> selects many more elements that just the <a data-link-type=element title=content>content</a> element.
		However, since only the <a data-link-type=element title=content>content</a> element has a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a>,
		it’s the only element that has a <a class=css data-link-type=maybe href=#selectordef-content title=::content>::content</a> pseudo-element as well.</p>
	</div>


<h4 class="heading settled" data-level=3.2.4 id=deep-combinator><span class=secno>3.2.4. </span><span class=content>
Selecting Through Shadows: the <span class=css data-link-for="">deep/</span> combinator</span><a class=self-link href=#deep-combinator></a></h4>

<p>When a <dfn class=css data-dfn-type=selector data-export="" id=selectorde
2A19
f--deep->/deep/<a class=self-link href=#selectordef--deep-></a></dfn> combinator is encountered in a selector,
	replace every element in the <a data-link-type=dfn href=http://dev.w3.org/csswg/selectors-4/#selector-match-list title="selector match list">selector match list</a>
	with every element reachable from the original element
	by traversing any number of child lists or shadow trees.</p>

	<div class=example>
		For example, say you had a component with a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> like the following:

		<pre>&lt;x-foo&gt;
  &lt;"shadow tree"&gt;
    &lt;div&gt;
      &lt;span id="not-top"&gt;...&lt;/span&gt;
    &lt;/div&gt;
    &lt;span id="top"&gt;...&lt;/span&gt;
    &lt;x-bar&gt;
      &lt;"shadow tree"&gt;
        &lt;span id="nested"&gt;...&lt;/span&gt;
      &lt;/&gt;
    &lt;/x-bar&gt;
  &lt;/&gt;
&lt;/x-foo&gt;
</pre>

<p>For a stylesheet in the outer document,
		the selector <span class=css>x-foo /deep/ span</span>
		selects all three of <code>&lt;span&gt;</code> elements:
		<span class=css>#top</span>, <span class=css>#not-top</span>, <em>and</em> <span class=css>#nested</span>.</p>
	</div>

	<p class=issue id=issue-dde39fe8><a class=self-link href=#issue-dde39fe8></a>
		This is basically a super-descendant combinator.
		If the descendant combinator had a real glyph,
		it would potentially be interesting to just double it.
		Maybe we can give the descendant combinator a pseudonym of <span class=css>&gt;&gt;</span>,
		as it itself is a super-child combinator?
		Then <span class=css data-link-for="">deep/</span> could be spelled <span class=css>&gt;&gt;&gt;</span>


E1A7
<h3 class="heading settled" data-level=3.3 id=shadow-cascading><span class=secno>3.3. </span><span class=content>
Shadow Cascading &amp; Inheritance</span><a class=self-link href=#shadow-cascading></a></h3>

<h4 class="heading settled" data-level=3.3.1 id=cascading><span class=secno>3.3.1. </span><span class=content>
Cascading</span><a class=self-link href=#cascading></a></h4>

<p>To address the desired cascading behavior of rules targetting elements in shadow roots,
	this specification extends the <a href=http://dev.w3.org/csswg/css-cascade/#cascading>cascade order</a>
	defined in the Cascade specification. <a data-biblio-type=normative data-link-type=biblio href=#biblio-css3cascade title=CSS3CASCADE>[CSS3CASCADE]</a></p>

<p>An additional cascade criteria must be added,
	between Origin and Scope,
	called Shadow Tree.</p>

	<ul>
		<li>
			When comparing two declarations,
			if one of them is in a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>
			and the other is in a document that contains that <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>,
			then for normal rules the declaration from the outer document wins,
			and for important rules the declaration from the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> wins.

<p class=note role=note>Note: This is the <em>opposite</em> of how scoped styles work.</p>

		<li>
			When comparing two declarations,
			if both are in <a data-link-type=dfn href=#shadow-tree title="shadow trees">shadow trees</a> with the same <a data-link-type=dfn href=#host-element title="host element">host element</a>,
			then for normal rules the declaration from the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> that was created most recently wins,
			and for important rules the declaration from the <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a> that was created less recently wins.
	</ul>

<p>When calculating <a href=http://dev.w3.org/csswg/css-cascade/#cascade-order>Order of Appearance</a>,
	the tree of trees,
	defined by the Shadow DOM specification,
	is used to calculate ordering.</p>

<h4 class="heading settled" data-level=3.3.2 id=inheritance><span class=secno>3.3.2. </span><span class=content>
Inheritance</span><a class=self-link href=#inheritance></a></h4>

<p>The <a data-link-type=dfn href=#top-level-element title="top-level elements">top-level elements</a> of a <a data-link-type=dfn href=#shadow-tree title="shadow tree">shadow tree</a>
	inherit from their <a data-link-type=dfn href=#host-element title="host element">host element</a>.</p>

<p>The elements in a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a> inherit from the parent of the <a data-link-type=element title=content>content</a> element they are ultimately distributed to,
	rather than from their normal parent.</p>


<h2 class="heading settled" data-level=4 id=fragment-scoping><span class=secno>4. </span><span class=content>
Fragmented Styling</span><a class=self-link href=#fragment-scoping></a></h2>

<p>Fragmented content can be styled differently
	based on which line, column, page, region, etc.
	it appears in.
	This is done by using an appropriate <dfn data-dfn-type=dfn data-export="" id=fragment-pseudo-element>fragment pseudo-element<a class=self-link href=#fragment-pseudo-element></a></dfn>,
	which allows targetting individual fragments of an element
	rather than the entire element.</p>

    <div class=example>
		In our example,
		the designer wants to make
		text flowing into #region1
		dark blue and bold.
		This design can be expressed as shown below.

		<pre>#region1::region p {
  color: #0C3D5F;
  font-weight: bold;
}
</pre>

<p>The <span class=css>::region</span> pseudo-element
		is followed by a <span class=css>p</span> relative selector in this example.
		The color and font-weight declarations will apply
		to any fragments of paragraphs that are
		displayed in <span class=css>#region1</span>.
		The following figure shows how
		the rendering changes
		if we apply this styling specific to <span class=css>#region1</span>.
		Note how less text fits into this box
		now that the <a class="property css" data-link-type=property href=http://dev.w3.org/csswg/css-fonts-3/#propdef-font-weight title=font-weight>font-weight</a> is
		bold instead of normal.<p></p>

		<figure>
			<img alt="Illustrate how changing region styling affects the flow of content." src=images/region-styling.png width=450>

			<figcaption>Different rendering with a different region styling</figcaption>
		</figure>
	</div>

<p class=note role=note>Note: This feature is an extension of <span class=css>::first-line</span> styling.</p>

<h3 class="heading settled" data-level=4.1 id=the-region-pseudo-element><span class=secno>4.1. </span><span class=content>
Region-based Styling: the <span class=css>::region</span> pseudo-element</span><a class=self-link href=#the-region-pseudo-element></a></h3>

	<div class=issue id=issue-266c17c8><a class=self-link href=#issue-266c17c8></a>
		Extend this to specify:
		<ul>
			<li><span class=css>&lt;region-element-selector&gt;::region</span>
			<li><span class=css>&lt;paginated-element-selector&gt;::page(&lt;page-selector&gt;)</span>
			<li><span class=css>&lt;multicol-element&gt;::column(&lt;AnB&gt;)</span>
			<li><span class=css>&lt;fragmented-element-selector&gt;::nth-fragment(&lt;AnB&gt;)</span>
			<li><span class=css>::first-line</span>
		</ul>
	</div>

    <p>A ::region pseudo-element represents a relationship between
      a selector that matches a <span>CSS Region</span>,
      and a relative selector that matches some named flow content.
      This allows style declarations to be applied
      to fragments of named flow content flowing
      into particular regions.</p>

        <pre>&lt;region selector&gt;::region &lt;content selector&gt;  {
    ... CSS styling declarations ...
}
</pre>

    <p>When the ::region pseudo-element is appended to a
    <a href=http://www.w3.org/TR/css3-selectors/#selector-syntax>selector</a>
    that matches one or more <span title="CSS Region">CSS Regions</span>,
    this creates a 'flow fragment' selector.
    The flow fragment selector specifies
    which range of elements in the flow
    can be matched by the relative selector.
    The relative selector can match elements
    in the range(s) (see <a data-biblio-type=normative data-link-type=biblio href=#biblio-dom title=DOM>[DOM]</a>) of the named flow
    that are displayed fully or partially
    in the selected region(s).</p>

    <p>Elements that are fully or partially
      in the flow fragment range may match the relative selector.
      However, the style declarations only apply
      to the fragment of the element
      that is displayed in the corresponding region(s).</p>

    <p>Only a limited list of properties apply to a ::region pseudo-element:</p>

    <p class=issue id=issue-9b00b7af><a class=self-link href=#issue-9b00b7af></a>
    	Either this list should be all functionally inheritable properties,
    	or all properties.
    	Why is it a seemingly-arbitrary subset of all properties, including box properties?

    <ol>
        <li><a href=http://www.w3.org/TR/CSS2/fonts.html>font properties</a></li>
        <li><a href=http://www.w3.org/TR/CSS2/colors.html>color property</a></li>
        <li><a href=http://www.w3.org/TR/css3-color/#transparency>opacity property</a></li>
        <li><a href=http://www.w3.org/TR/css3-background/#backgrounds>background property</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-text-3/#propdef-word-spacing title=word-spacing>word-spacing</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-text-3/#propdef-letter-spacing title=letter-spacing>letter-spacing</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-text-decor-3/#propdef-text-decoration title=text-decoration>text-decoration</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-text-3/#propdef-text-transform title=text-transform>text-transform</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visudet.html#propdef-line-height title=line-height>line-height</a></li>
        <li><a href=http://www.w3.org/TR/css3-text/#justification>alignment and justification properties</a></li>
        <li><a href=http://www.w3.org/TR/css3-background/#borders>border properties</a></li>
        <li><a href=http://www.w3.org/TR/css3-background/#corners>rounded corner properties</a></li>
        <li><a href=http://www.w3.org/TR/css3-background/#border-images>border images properties</a></li>
        <li><a href=http://www.w3.org/TR/CSS2/box.html#margin-properties>margin properties</a></li>
        <li><a href=http://www.w3.org/TR/CSS2/box.html#padding-properties>padding properties</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-text-decor-3/#propdef-text-shadow title=text-shadow>text-shadow</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-backgrounds-3/#box-shadow title=box-shadow>box-shadow</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-break-3/#box-decoration-break title=box-decoration-break>box-decoration-break</a></li>
        <li><a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visudet.html#propdef-width title=width>width</a></li>
    </ol>

    <div class=example id=region-style-example><a class=self-link href=#region-style-example></a>
        <p>In the following example, the <span>named flow</span> “article-flow” flows
        into “region-1” and “region-2”.</p>
        <pre>&lt;style&gt;
  #div-1 {
    <strong>flow-into: article-flow;</strong>
  }

  #region-1, #region-2 {
    <strong>flow-from: article-flow;</strong>
  }

  /* region styling */
  <strong>#region-1::region p </strong> {
    margin-right: 5em;
  }
&lt;/style&gt;

&lt;body&gt;
  &lt;div id="div-1"&gt;
      &lt;p id="p-1"&gt;...&lt;/p&gt;
      &lt;p id="p-2"&gt;...&lt;/p&gt;
  &lt;/div&gt;
  &lt;div id="region-1"&gt;&lt;/div&gt;
  &lt;div id="region-2"&gt;&lt;/div&gt;
&lt;/body&gt;
</pre>

        <div id=region_styling_illustration>
            <img alt="Example showing how a named flow content fits into regions to illustrate region styling." id=region_styling_img_2 src=images/region-styling-2.png>

            <ul class=swatch-list>
                <li><span class=swatch style=background:#1C75BC> </span>div div-1</li>

                <li><span class=swatch style=background:crimson> </span>paragraph p-1</li>

                <li><span class=swatch style=background:white> </span>paragraph p-2</li>

                <li><span class=swatch style=background:#E6E7E8> </span>range of flow that fits into
                region-1</li>

                <li><span class=swatch style=background:#BCBEC0> </span>range of flow that fits into
                region-2</li>
            </ul>
        </div>

        <p>The region styling applies
          to flow content that fits in <span class=css>region-1</span>.
          The relative selector matches <span class=css>p-1</span> and <span class=css>p-2</span>
          because these paragraphs
          flow into <span class=css>region-1</span>.
          Only the fragment of <span class=css>p-2</span>
          that flows into <span class=css>region-1</span>
          is styled with the pseudo-element.</p>
    </div>

    <p>All of the selectors
      in a ::region pseudo-element
      contribute to its <a href=http://www.w3.org/TR/css3-selectors/#specificity>specificity</a>.
      So the specificity of the ::region pseudo-element
      in the example above would combine
      the id selector’s specificity
      with the specificity of the type selector,
      resulting in a specificity of 101.</p>

    <p>Selectors that match a given element or element fragment (as described above),
    participate in the <a href=http://www.w3.org/TR/2011/REC-CSS2-20110607/cascade.html#cascading-order>CSS Cascading
    order</a> as defined in <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>.</p>

    <div>
        Region styling does not apply to nested regions. For example, if a region
        <span class=css>A</span> receives content from a flow that contains region <span class=css>B</span>, the content that
        flows into <span class=css>B</span> does not receive the region styling specified for region <span class=css>A</span>.
    </div>

    <p class=issue id=issue-5bff6297><a class=self-link href=#issue-5bff6297></a>
    	We’ll need some way to query the styles of a fragment in a particular region.
    	<code>getComputedStyle()</code> isn’t enough,
    	because an element can exist in multiple regions, for example,
    	with each fragment receiving different styles.

</main>
<h2 class="no-ref no-num heading settled" id=conformance><span class=content>
Conformance</span><a class=self-link href=#conformance></a></h2>

<h3 class="no-ref heading settled" 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=#biblio-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 role=note>Note, this is an informative note.</p>

    <p>Advisements are normative sections styled to evoke special attention and are
    set apart from other normative text with <code>&lt;strong class="advisement"&gt;</code>, like
    this:

    <strong class=advisement>
        UAs MUST provide an accessible alternative.
    </strong>

<h3 class="no-ref heading settled" 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" 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>