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<!doctype html><html lang="en">
<head>
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<title>CSS Shadow Parts</title>
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<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/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
<h1 class="p-name no-ref" id="title">CSS Shadow Parts</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Unofficial Proposal Draft, <time class="dt-updated" datetime="2017-01-20">20 January 2017</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="http://tabatkins.github.io/specs/css-shadow-parts/">http://tabatkins.github.io/specs/css-shadow-parts/</a>
<dt>Issue Tracking:
<dd><a href="#issues-index">Inline In Spec</a>
<dd><span><a href="https://github.com/w3c/csswg-drafts/labels/css-shadow-parts-1">GitHub Issues</a></span>
<dt class="editor">Editor:
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://xanthir.com/contact/">Tab Atkins-Bittner</a> (<span class="p-org org">Google</span>)
</dl>
</div>
<div data-fill-with="warning">
<details class="annoying-warning" open="">
<summary>Not Ready For Implementation</summary>
<p> This spec is not yet ready for implementation.
It exists in this repository to record the ideas and promote discussion. </p>
<p> Before attempting to implement this spec,
please contact the CSSWG at www-style@w3.org. </p>
</details>
</div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2017 <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>). 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 the <a class="css" data-link-type="maybe" href="#selectordef-part">::part()</a> and <a class="css" data-link-type="maybe" href="#selectordef-theme">::theme()</a> pseudo-elements on <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow hosts</a>, allowing <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow hosts</a> to selectively expose chosen elements from their <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a> to the outside page for styling purposes.</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></p>
</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="#motivation"><span class="secno">1.1</span> <span class="content">Motivation</span></a>
</ol>
<li><a href="#part-attr"><span class="secno">2</span> <span class="content">Exposing a Shadow Element: the <code><span>part</span></code> attribute</span></a>
<li><a href="#part-theme"><span class="secno">3</span> <span class="content">Selecting a Shadow Element: the <span class="css">::part()</span> and <span class="css">::theme()</span> pseudo-elements</span></a>
<li>
<a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
<ol class="toc">
<li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
<li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
<li>
<a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
<ol class="toc">
<li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
<li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
<li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
</ol>
</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>
</ol>
<li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues 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 class="issue" id="issue-0b399633"><a class="self-link" href="#issue-0b399633"></a> This spec is intentionally a rough sketch at the moment.
It should contain all the details necessary to evaluate the proposal,
but is intentionally avoiding precise algorithms at the moment,
to aid in easy comprehension
and to, hopefully, discourage implementation from this sketch.</p>
<p>Shadow DOM allows authors to separate their page into "components",
subtrees of markup whose details are only relevant to the component itself,
not the outside page.
This reduces the chance of a style meant for one part of the page
accidentally over-applying and making a different part of the page look wrong.
However, this styling barrier also makes it harder for a page to interact with its components
when it actually <em>wants</em> to do so.</p>
<p>This specification defines the <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-1">::part()</a> and <a class="css" data-link-type="maybe" href="#selectordef-theme" id="ref-for-selectordef-theme-1">::theme()</a> pseudo-elements,
which allow an author to style specific, purposely exposed elements in a <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a> from the outside page’s context.
In combination with <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a>,
which let the outside page pass particular values
(such as theme colors)
into the component for it to do with as it will,
these pseudo-elements allow components and the outside page
to interact in safe, powerful ways,
maintaining encapsulation
without surrending all control.</p>
<h3 class="heading settled" data-level="1.1" id="motivation"><span class="secno">1.1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h3>
<p>For obvious reasons,
it’s valuable to let the outside page style the internals of a shadow tree,
at least in some limited ways.
(The ubiquity of UA-specific pseudo-elements for the various input elements shows this.)</p>
<p>The previous proposed method for doing so,
the >>> combinator,
turned out to be <em>too powerful</em> for its own good;
it exposed too much of a component’s internal structure to scrutiny,
defeating some of the encapsulation benefits that using Shadow DOM brings.
For this,
and other performance-related reasons,
the >>> combinator was eventually removed from the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#dynamic-profile">dynamic profile</a>.</p>
<p>This left us with using <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a> as the only way to style into a shadow tree:
the component would advertise that it uses certain <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a> to style its internals,
and the outer page could then set those properties as it wished on the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>,
letting inheritance push the values down to where they were needed.
This works very well for many simple theming use-cases.</p>
<p>However, there are some cases where this falls down.
If a component wishes to allow arbitrary styling of something in its shadow tree,
the only way to do so is to define hundreds of <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a> (one per CSS property they wish to allow control of),
which is obviously ridiculous
for both usability and performance reasons.
The situation is compounded if authors wish to style the component differently
based on pseudo-classes like <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/selectors-4/#hover-pseudo">:hover</a>;
the component needs to duplicate the <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a> used
for each pseudo-class
(and each combination,
like <span class="css">:hover:focus</span>,
resulting in a combinatorial explosion).
This makes the usability and performance problems even worse.</p>
<p>We introduce <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-2">::part()</a> to handle this case much more elegantly and performantly.
Rather than bundling everything into <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom property</a> names,
the functionality lives in selectors and style rule syntax,
like it’s meant to.
This is far more usable for both component authors
and component users,
should have much better performance,
and allows for better encapsulation/API surface.</p>
<p>Another interesting facet of using <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a>,
however,
is that inheritance doesn’t stop at the first shadow tree.
Unless explicitly blocked,
a <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom property</a> inherits down thru nested trees,
allowing authors to style deeply nested components
as easily as they style directly-visible ones.
The same considerations apply to this case,
so we introduce <a class="css" data-link-type="maybe" href="#selectordef-theme" id="ref-for-selectordef-theme-2">::theme()</a> to handle this.</p>
<p>It’s important to note that <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-3">::part()</a> and <a class="css" data-link-type="maybe" href="#selectordef-theme" id="ref-for-selectordef-theme-3">::theme()</a> offer <em>absolutely zero new theoretical power</em>.
They are not a rehash of the <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-scoping-1/#selectordef-shadow-piercing-descendant-combinator">>>></a> combinator,
they’re simply a more convenient and consistent syntax
for something authors can already do with <a data-link-type="dfn" href="https://drafts.csswg.org/css-variables-1/#custom-property">custom properties</a>.
By separating out the explicitly "published" parts of an element
(the <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-1">shadow part map</a> from the sub-parts that it merely happens to contain
(the <a data-link-type="dfn" href="#shadow-root-shadow-theme-map" id="ref-for-shadow-root-shadow-theme-map-1">shadow theme map</a>,
it also helps with encapsulation,
as authors can use <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-4">::part()</a> without fear of accidental over-styling.</p>
<h2 class="heading settled" data-level="2" id="part-attr"><span class="secno">2. </span><span class="content">Exposing a Shadow Element: the <code><a data-link-type="element-sub" href="#element-attrdef-html-global-part" id="ref-for-element-attrdef-html-global-part-1">part</a></code> attribute</span><a class="self-link" href="#part-attr"></a></h2>
<p>Any element in a shadow tree can have a <dfn class="dfn-paneled" data-dfn-for="html-global" data-dfn-type="element-attr" data-export="" id="element-attrdef-html-global-part">part</dfn> attribute.
This is used to expose the element outside the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>,
and to "forward" sub-parts of the element
(if it has its own <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>)
to outside the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>.</p>
<p>The part attribute is parsed as a comma-separated list of part mappings.
Each part mapping is one of:</p>
<dl class="switch">
<dt data-md="">
<p><code>ident</code></p>
<dd data-md="">
<p>Adds «[ ident → el ]» to the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-2">shadow part map</a>.</p>
<dt data-md="">
<p><code>ident1 => ident2</code></p>
<dd data-md="">
<p>If el is a <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>,
and it’s <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-3">shadow part map</a> <var>partMap</var> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#map-exists">contains</a> ident1,
then this adds «[ ident2 → <var>partMap</var>[ident1] ]» to the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-4">shadow part map</a>.</p>
<dt data-md="">
<p><code>* => prefix*</code></p>
<dd data-md="">
<p>If el is a <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>,
then <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#map-iterate">for each</a> <var>ident</var> → <var>subEl</var> in el’s <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-5">shadow part map</a>,
«[ prefix + <var>ident</var> → <var>subEl</var> ]» is added to the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-6">shadow part map</a>.</p>
<dt data-md="">
<p>anything else</p>
<dd data-md="">
<p>Ignored for error-recovery / future compat.</p>
</dl>
<p class="note" role="note">Note: It’s okay to give a part multiple names,
or map a sub-part to several names.
The "part name" should be considered similar to a class,
not an id or tagname.</p>
<p>Each <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</a> has a <dfn class="dfn-paneled" data-dfn-for="shadow root" data-dfn-type="dfn" data-export="" id="shadow-root-shadow-part-map">shadow part map</dfn> and a <dfn class="dfn-paneled" data-dfn-for="shadow root" data-dfn-type="dfn" data-export="" id="shadow-root-shadow-theme-map">shadow theme map</dfn>,
both of which are <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-map">ordered maps</a>.</p>
<p>The <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-7">shadow part map</a> contains all the entries described by the elements in its <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>,
as described above.</p>
<p>If the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</a>’s <code class="idl"><a data-link-type="idl" href="https://dom.spec.whatwg.org/#dom-shadowroot-mode">mode</a></code> is <code class="idl"><a data-link-type="idl" href="https://dom.spec.whatwg.org/#dom-shadowrootmode-closed">"closed"</a></code>,
the <a data-link-type="dfn" href="#shadow-root-shadow-theme-map" id="ref-for-shadow-root-shadow-theme-map-2">shadow theme map</a> is identical to the <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-8">shadow part map</a>.
Otherwise,
it’s the concatenation of the <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-9">shadow part map</a> with the <a data-link-type="dfn" href="#shadow-root-shadow-theme-map" id="ref-for-shadow-root-shadow-theme-map-3">shadow theme maps</a> of every <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>’s <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</a> in its <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>.</p>
<p class="issue" id="issue-4303394f"><a class="self-link" href="#issue-4303394f"></a> TODO: Define a syntax or new attribute
to prevent an element from adding things to its <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-theme-map" id="ref-for-shadow-root-shadow-theme-map-4">shadow theme map</a>.</p>
<h2 class="heading settled" data-level="3" id="part-theme"><span class="secno">3. </span><span class="content">Selecting a Shadow Element: the <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-5">::part()</a> and <a class="css" data-link-type="maybe" href="#selectordef-theme" id="ref-for-selectordef-theme-4">::theme()</a> pseudo-elements</span><a class="self-link" href="#part-theme"></a></h2>
<p>The <dfn class="dfn-paneled css" data-dfn-type="selector" data-export="" id="selectordef-part">::part()</dfn> and <dfn class="dfn-paneled css" data-dfn-type="selector" data-export="" id="selectordef-theme">::theme()</dfn> pseudo-elements
(collectively, the <dfn class="dfn-paneled" data-dfn-type="dfn" data-export="" id="shadow-part-pseudo-elements">shadow-part pseudo-elements</dfn>)
allow you to select elements that have been exposed via a <code><a data-link-type="element-sub" href="#element-attrdef-html-global-part" id="ref-for-element-attrdef-html-global-part-2">part</a></code> attribute.
The syntaxes of them are:</p>
<pre class="prod">::part() = ::part( <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a> )
::theme() = ::theme( <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a> )
</pre>
<p>The <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-6">::part()</a> pseudo-element only matches anything
when the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#originating-element">originating element</a> is a <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>.
If the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#originating-element">originating element’s</a> <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-10">shadow part map</a> <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#map-exists">contains</a> the specified <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a>, <a class="css" data-link-type="maybe" href="#selectordef-part" id="ref-for-selectordef-part-7">::part()</a> matches the element or elements keyed to that <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a>.
Otherwise, it matches nothing.</p>
<div class="example" id="example-d833c64c"><a class="self-link" href="#example-d833c64c"></a> For example,
if you have a custom button
that contains a "label" element that is exposed for styling
(via <code>part="label"</code>),
you can select it with <span class="css">#the-button::part(label)</span>. </div>
<p>The <a class="css" data-link-type="maybe" href="#selectordef-theme" id="ref-for-selectordef-theme-5">::theme()</a> pseudo-element is similar,
except it can match regardless of whether the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#originating-element">originating element</a> is a <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a> or not.
It matches the elements keyed to the specified <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a> in the <a data-link-type="dfn" href="#shadow-root-shadow-theme-map" id="ref-for-shadow-root-shadow-theme-map-5">shadow theme map</a> of the <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow trees</a> of the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#originating-element">originating element</a> or any descendants.</p>
<div class="example" id="example-860e4f35"><a class="self-link" href="#example-860e4f35"></a> For example, <span class="css">:root::theme(label)</span> matches any element with <code>part="label"</code> anywhere in the entire document,
no matter how deeply nested into shadow trees they are. </div>
<p>The <a data-link-type="dfn" href="#shadow-part-pseudo-elements" id="ref-for-shadow-part-pseudo-elements-1">shadow-part pseudo-elements</a> can take additional pseudo-classes after them,
such as <span class="css">x-button::part(label):hover</span>,
but never match the <a data-link-type="dfn" href="https://drafts.csswg.org/selectors-4/#structural-pseudo-classes">structural pseudo-classes</a> or any other pseudo-classes that match based on tree information
rather than local element information.</p>
<p>The <a data-link-type="dfn" href="#shadow-part-pseudo-elements" id="ref-for-shadow-part-pseudo-elements-2">shadow-part pseudo-elements</a> also can take additional pseudo-elements after them,
such as <span class="css">x-button::part(label)::before</span>,
but never match additional <a data-link-type="dfn" href="#shadow-part-pseudo-elements" id="ref-for-shadow-part-pseudo-elements-3">shadow-part pseudo-elements</a>.</p>
<div class="example" id="example-13b4219d">
<a class="self-link" href="#example-13b4219d"></a> For example, <span class="css">x-panel::part(confirm-button)::part(label)</span> never matches anything.
This is because doing so would expose more structural information
than is intended.
<p>One can still target the nested label with a selector like <span class="css">x-panel::theme(label)</span>.
However, this will also select the labels of any other buttons in the panel.</p>
<p>If the <code><x-panel></code>’s internal confirm button had used something like <code>part="confirm-button, * => confirm-*"</code> to forward the button’s internal parts up into the panel’s own <a data-link-type="dfn" href="#shadow-root-shadow-part-map" id="ref-for-shadow-root-shadow-part-map-11">shadow part map</a>,
then a selector like <span class="css">x-panel::part(confirm-label)</span> would select just the one button’s label,
ignoring any other labels.</p>
</div>
</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="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-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>
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-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: </p>
<div class="example" id="example-52448c84">
<a class="self-link" href="#example-52448c84"></a>
<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>
<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><strong class="advisement"></code>, like
this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
<h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
<p>Conformance to this specification
is defined for three conformance classes: </p>
<dl>
<dt>style sheet
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
style sheet</a>.
<dt>renderer
<dd>A <a href="http://www.w3.org/TR/CSS2/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/CSS2/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>
<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>
<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. </p>
<h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
<p>The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future. </p>
<h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
<p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
(and <a href="http://www.w3.org/TR/CSS2/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</strong>.
In particular, user agents <em>must not</em> selectively ignore
unsupported property 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>
<h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
<p>To avoid clashes with future stable CSS features,
the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
<h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>
<p>Once a specification reaches the Candidate Recommendation stage,
implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature. </p>
<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>
<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.
<script src="https://www.w3.org/scripts/TR/2016/fixup.js"></script>
</p>
<h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
<h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
<ul class="index">
<li><a href="#selectordef-part">::part()</a><span>, in §3</span>
<li><a href="#element-attrdef-html-global-part">part</a><span>, in §2</span>
<li><a href="#shadow-root-shadow-part-map">shadow part map</a><span>, in §2</span>
<li><a href="#shadow-part-pseudo-elements">shadow-part pseudo-elements</a><span>, in §3</span>
<li><a href="#shadow-root-shadow-theme-map">shadow theme map</a><span>, in §2</span>
<li><a href="#selectordef-theme">::theme()</a><span>, in §3</span>
</ul>
<h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
<ul class="index">
<li>
<a data-link-type="biblio">[css-scoping-1]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-scoping-1/#selectordef-shadow-piercing-descendant-combinator">>>></a>
</ul>
<li>
<a data-link-type="biblio">[css-values-3]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-values-4/#typedef-ident"><ident></a>
</ul>
<li>
<a data-link-type="biblio">[css-variables-1]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/css-variables-1/#custom-property">custom property</a>
</ul>
<li>
<a data-link-type="biblio">[WHATWG-DOM]</a> defines the following terms:
<ul>
<li><a href="https://dom.spec.whatwg.org/#dom-shadowrootmode-closed">"closed"</a>
<li><a href="https://dom.spec.whatwg.org/#dom-shadowroot-mode">mode</a>
<li><a href="https://dom.spec.whatwg.org/#element-shadow-host">shadow host</a>
<li><a href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root</a>
<li><a href="https://dom.spec.whatwg.org/#concept-shadow-tree">shadow tree</a>
</ul>
<li>
<a data-link-type="biblio">[INFRA]</a> defines the following terms:
<ul>
<li><a href="https://infra.spec.whatwg.org/#map-exists">contains</a>
<li><a href="https://infra.spec.whatwg.org/#map-iterate">for each</a>
<li><a href="https://infra.spec.whatwg.org/#ordered-map">ordered map</a>
</ul>
<li>
<a data-link-type="biblio">[selectors-4]</a> defines the following terms:
<ul>
<li><a href="https://drafts.csswg.org/selectors-4/#hover-pseudo">:hover</a>
<li><a href="https://drafts.csswg.org/selectors-4/#dynamic-profile">dynamic profile</a>
<li><a href="https://drafts.csswg.org/selectors-4/#originating-element">originating element</a>
<li><a href="https://drafts.csswg.org/selectors-4/#structural-pseudo-classes">structural pseudo-classes</a>
</ul>
</ul>
<h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
<h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
<dl>
<dt id="biblio-css-scoping-1">[CSS-SCOPING-1]
<dd>Tab Atkins Jr.; Elika Etemad. <a href="http://dev.w3.org/csswg/css-scoping/">CSS Scoping Module Level 1</a>. URL: <a href="http://dev.w3.org/csswg/css-scoping/">http://dev.w3.org/csswg/css-scoping/</a>
<dt id="biblio-css-values-3">[CSS-VALUES-3]
<dd>Tab Atkins Jr.; Elika Etemad. <a href="https://drafts.csswg.org/css-values/">CSS Values and Units Module Level 3</a>. URL: <a href="https://drafts.csswg.org/css-values/">https://drafts.csswg.org/css-values/</a>
<dt id="biblio-css-variables-1">[CSS-VARIABLES-1]
<dd>Tab Atkins Jr.. <a href="http://dev.w3.org/csswg/css-variables/">CSS Custom Properties for Cascading Variables Module Level 1</a>. URL: <a href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a>
<dt id="biblio-infra">[INFRA]
<dd>Anne van Kesteren; Domenic Denicola. <a href="https://infra.spec.whatwg.org/">Infra Standard</a>. Living Standard. URL: <a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
<dt id="biblio-rfc2119">[RFC2119]
<dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
<dt id="biblio-selectors-4">[SELECTORS-4]
<dd>Selectors Level 4 URL: <a href="https://drafts.csswg.org/selectors-4/">https://drafts.csswg.org/selectors-4/</a>
<dt id="biblio-whatwg-dom">[WHATWG-DOM]
<dd>Anne van Kesteren. <a href="https://dom.spec.whatwg.org/">DOM Standard</a>. Living Standard. URL: <a href="https://dom.spec.whatwg.org/">https://dom.spec.whatwg.org/</a>
</dl>
<h2 class="no-num no-ref 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"> This spec is intentionally a rough sketch at the moment.
It should contain all the details necessary to evaluate the proposal,
but is intentionally avoiding precise algorithms at the moment,
to aid in easy comprehension
and to, hopefully, discourage implementation from this sketch.<a href="#issue-0b399633"> ↵ </a></div>
<div class="issue"> TODO: Define a syntax or new attribute
to prevent an element from adding things to its <a data-link-type="dfn" href="https://dom.spec.whatwg.org/#concept-shadow-root">shadow root’s</a> <a data-link-type="dfn" href="#shadow-root-shadow-theme-map">shadow theme map</a>.<a href="#issue-4303394f"> ↵ </a></div>
</div>
<aside class="dfn-panel" data-for="element-attrdef-html-global-part">
<b><a href="#element-attrdef-html-global-part">#element-attrdef-html-global-part</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-element-attrdef-html-global-part-1">2. Exposing a Shadow Element: the part attribute</a>
<li><a href="#ref-for-element-attrdef-html-global-part-2">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a>
</ul>
</aside>
<aside class="dfn-panel" data-for="shadow-root-shadow-part-map">
<b><a href="#shadow-root-shadow-part-map">#shadow-root-shadow-part-map</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-shadow-root-shadow-part-map-1">1.1. Motivation</a>
<li><a href="#ref-for-shadow-root-shadow-part-map-2">2. Exposing a Shadow Element: the part attribute</a> <a href="#ref-for-shadow-root-shadow-part-map-3">(2)</a> <a href="#ref-for-shadow-root-shadow-part-map-4">(3)</a> <a href="#ref-for-shadow-root-shadow-part-map-5">(4)</a> <a href="#ref-for-shadow-root-shadow-part-map-6">(5)</a> <a href="#ref-for-shadow-root-shadow-part-map-7">(6)</a> <a href="#ref-for-shadow-root-shadow-part-map-8">(7)</a> <a href="#ref-for-shadow-root-shadow-part-map-9">(8)</a>
<li><a href="#ref-for-shadow-root-shadow-part-map-10">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a> <a href="#ref-for-shadow-root-shadow-part-map-11">(2)</a>
</ul>
</aside>
<aside class="dfn-panel" data-for="shadow-root-shadow-theme-map">
<b><a href="#shadow-root-shadow-theme-map">#shadow-root-shadow-theme-map</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-shadow-root-shadow-theme-map-1">1.1. Motivation</a>
<li><a href="#ref-for-shadow-root-shadow-theme-map-2">2. Exposing a Shadow Element: the part attribute</a> <a href="#ref-for-shadow-root-shadow-theme-map-3">(2)</a> <a href="#ref-for-shadow-root-shadow-theme-map-4">(3)</a>
<li><a href="#ref-for-shadow-root-shadow-theme-map-5">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a>
</ul>
</aside>
<aside class="dfn-panel" data-for="selectordef-part">
<b><a href="#selectordef-part">#selectordef-part</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-selectordef-part-1">1. Introduction</a>
<li><a href="#ref-for-selectordef-part-2">1.1. Motivation</a> <a href="#ref-for-selectordef-part-3">(2)</a> <a href="#ref-for-selectordef-part-4">(3)</a>
<li><a href="#ref-for-selectordef-part-5">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a> <a href="#ref-for-selectordef-part-6">(2)</a> <a href="#ref-for-selectordef-part-7">(3)</a>
</ul>
</aside>
<aside class="dfn-panel" data-for="selectordef-theme">
<b><a href="#selectordef-theme">#selectordef-theme</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-selectordef-theme-1">1. Introduction</a>
<li><a href="#ref-for-selectordef-theme-2">1.1. Motivation</a> <a href="#ref-for-selectordef-theme-3">(2)</a>
<li><a href="#ref-for-selectordef-theme-4">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a> <a href="#ref-for-selectordef-theme-5">(2)</a>
</ul>
</aside>
<aside class="dfn-panel" data-for="shadow-part-pseudo-elements">
<b><a href="#shadow-part-pseudo-elements">#shadow-part-pseudo-elements</a></b><b>Referenced in:</b>
<ul>
<li><a href="#ref-for-shadow-part-pseudo-elements-1">3. Selecting a Shadow Element: the ::part() and ::theme() pseudo-elements</a> <a href="#ref-for-shadow-part-pseudo-elements-2">(2)</a> <a href="#ref-for-shadow-part-pseudo-elements-3">(3)</a>
</ul>
</aside>
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