Tweak <meta name=color-scheme> definition.

Depends on w3c/csswg-drafts#6731
Fixes #7213.

source

@@ -3778,6 +3778,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
     <ul class="brief">
      <li><dfn data-x-href="https://drafts.csswg.org/css-color-adjust/#color-scheme-prop">'color-scheme'</dfn></li>
+     <li><dfn data-x-href="https://drafts.csswg.org/css-color-adjust/#pages-supported-color-schemes">page's supported color-schemes</dfn></li>
     </ul>
 
     <p>The following term is defined in <cite>CSS Pseudo-Elements</cite>: <ref spec=CSSPSEUDO></p>
@@ -14589,8 +14590,7 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
     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>
+    property value. It determines the <span>page's supported color-schemes</span>.</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
@@ -14606,7 +14606,7 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
 
     <div w-nodev>
 
-     <p>To obtain a page's color-scheme, user agents must run the following steps:</p>
+     <p>To obtain a <span>page's supported color-schemes</span>, user agents must run the following steps:</p>
 
      <ol>
       <li>
@@ -14631,11 +14631,14 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
         <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>
+        return <var>parsed</var>.</li>
        </ol>
       </li>
+
+      <li>
+       <p>Return null.</p>
+      </li>
+
      </ol>
 
      <p>If any <code>meta</code> elements are <span data-x="node is inserted into a