Specify <meta name=color-scheme>

The specification is currently in the CSS Color Adjustment spec [1]. It
was resolved [2] in the CSSWG that it should be moved to the HTML spec,
with a reference [3] from CSS Color Adjustment.

[1]: https://drafts.csswg.org/css-color-adjust/#color-scheme-meta
[2]: w3c/csswg-drafts#3846 (comment)
[3]: w3c/csswg-drafts#3846 (comment)

source

@@ -3615,6 +3615,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
     <ul class="brief">
      <li><dfn data-x-href="https://drafts.csswg.org/css-syntax/#conform-classes">conformant style sheet</dfn></li>
+     <li><dfn data-x-href="https://drafts.csswg.org/css-syntax/#parse-a-list-of-component-values">parse a list of component values</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/css-syntax/#parse-a-comma-separated-list-of-component-values">parse a comma-separated list of component values</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/css-syntax/#component-value">component value</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/css-syntax/#environment-encoding">environment encoding</dfn></li>
@@ -3685,6 +3686,13 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <ul class="brief">
      <li><dfn data-x-href="https://drafts.csswg.org/css-scoping/#flat-tree">flat tree</dfn></li>
     </ul>
+
+    <p>The following terms and features are defined in <cite>CSS Color Adjustment</cite>: <ref
+    spec=CSSCOLORADJUST></p>
+
+    <ul class="brief">
+     <li><dfn data-x-href="https://drafts.csswg.org/css-color-adjust/#color-scheme-prop">'color-scheme'</dfn></li>
+    </ul>
    </dd>
 
 
@@ -14220,6 +14228,77 @@ interface <dfn>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
     the theme color as a background and display white text over it, it might use a darker variant of
     the theme color in that part of the UI, to ensure adequate contrast.</p>
    </dd>
+
+   <dt><dfn><code data-x="meta-color-scheme">color-scheme</code></dfn></dt>
+
+   <dd>
+    <p>To aid user agents in rendering the page background with the desired color scheme immediately
+    (rather than waiting for all CSS in the page to load), a <span>'color-scheme'</span> value can
+    be provided in a <code>meta</code> element.</p>
+
+    <p>The value must be a string that matches the syntax for the CSS <span>'color-scheme'</span>
+    property value, and will be cascaded with the color-scheme declarations for the document root
+    element as described below.</p>
+
+    <p>There must not be more than one <code>meta</code> element with its <code
+    data-x="attr-meta-name">name</code> attribute value set to an
+    <span>ASCII case-insensitive</span> match for <code
+    data-x="meta-color-scheme">color-scheme</code> per document.</p>
+
+    <div class="example">
+     <p>The following declaration indicates that the page is aware of and can handle a color scheme
+     with dark background colors and light foreground colors:</p>
+
+     <pre><code class="html">&lt;meta name="color-scheme" content="dark"></code></pre>
+    </div>
+
+    <div w-nodev>
+
+     <p>To obtain a page's color-scheme, user agents must run the following steps:</p>
+
+     <ol>
+      <li>
+       <p>Let <var>candidate elements</var> be the list of all <code>meta</code> elements that
+       meet the following criteria, in <span>tree order</span>:</p>
+
+       <ul class="brief">
+        <li>The element is <span>in a document tree</span></li>
+        <li>The element has a <code data-x="attr-meta-name">name</code> attribute, whose value is
+        an <span>ASCII case-insensitive</span> match for
+        <code data-x="meta-color-scheme">color-scheme</code></li>
+        <li>The element has a <code data-x="attr-meta-content">content</code> attribute</li>
+       </ul>
+      </li>
+
+      <li>
+       <p>For each <var>element</var> in <var>candidate elements</var>:</p>
+
+       <ol>
+        <li>Let <var>parsed</var> be the result of <span data-x="parse a list of component
+        values">parsing a list of component values</span> given the value of <var>element</var>'s
+        <code data-x="attr-meta-content">content</code> attribute.</li>
+
+        <li>If <var>parsed</var> is a valid CSS <span>'color-scheme'</span> property value, then
+        treat that value as a declaration of the CSS <span>'color-scheme'</span> property on
+        <var>element</var>'s <span>root</span>, cascaded as a <span
+        data-x="presentational hints">presentational hint</span>, and return.</li>
+       </ol>
+      </li>
+     </ol>
+
+     <p>If any <code>meta</code> elements are <span data-x="node is inserted into a
+     document">inserted into the document</span> or <span data-x="node is removed from a
+     document">removed from the document</span>, or existing <code>meta</code> elements have their
+     <code data-x="attr-meta-name">name</code> or <code data-x="attr-meta-content">content</code>
+     attributes changed, user agents must re-run the above algorithm.</p>
+
+    </div>
+
+    <p class="note">Because these rules check successive elements until they find a match, an
+    author can provide multiple such values to handle fallback for legacy user agents. Opposite to
+    how CSS fallback works for properties, the multiple meta elements needs to be arranged with the
+    legacy values after the newer values.</p>
+   </dd>
   </dl>
 
 
@@ -122795,6 +122874,9 @@ INSERT INTERFACES HERE
    <dt id="refsCSSCOLOR">[CSSCOLOR]</dt>
    <dd><cite><a href="https://drafts.csswg.org/css-color/">CSS Color Module</a></cite>, T. &Ccedil;elik, C. Lilley, L. Baron. W3C.</dd>
 
+   <dt id="refsCSSCOLORADJUST">[CSSCOLORADJUST]</dt>
+   <dd><cite><a href="https://drafts.csswg.org/css-color-adjust/">CSS Color Adjustment Module</a></cite>, E. Etemad, R. Atanassov, R. Lillesveen, T. Atkins. W3C.</dd>
+
    <dt id="refsCSSDISPLAY">[CSSDISPLAY]</dt>
    <dd><cite><a href="https://drafts.csswg.org/css-display/">CSS Display</a></cite>, T. Atkins, E. Etemad. W3C.</dd>