Add <meta name="responsive-embedded-sizing">

source

@@ -4008,6 +4008,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing/#fit-content-inline-size">fit-content inline size</dfn></li>
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing-4/#aspect-ratio">'aspect-ratio'</dfn> property</li>
       <li><dfn data-x-href="https://drafts.csswg.org/css-sizing/#intrinsic-size">intrinsic size</dfn></li>
+      <li><dfn data-x-href="https://drafts.csswg.org/css-sizing-4/#responsive-iframes">responsively-sized iframes</dfn></li>
     </ul>
 
     <p>The following features are defined in <cite>CSS Lists and Counters</cite>.
@@ -4469,6 +4470,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x="csp-EnsureCSPDoesNotBlockStringCompilation" data-x-href="https://w3c.github.io/webappsec-csp/#can-compile-strings">EnsureCSPDoesNotBlockStringCompilation</dfn> abstract operation</li>
      <li>The <dfn data-x-href="https://w3c.github.io/webappsec-csp/#allow-base-for-document">Is base allowed for Document?</dfn> algorithm</li>
      <li>The <dfn data-x-href="https://w3c.github.io/webappsec-csp/#frame-ancestors"><code data-x="">frame-ancestors</code> directive</dfn></li>
+     <li>The <dfn data-x-href="https://w3c.github.io/webappsec-csp/#grammardef-ancestor-source-list">ancestor-source-list</dfn> syntax</li>
      <li>The <dfn data-x-href="https://w3c.github.io/webappsec-csp/#sandbox"><code data-x="">sandbox</code> directive</dfn></li>
      <li>The <dfn data-x-href="https://w3c.github.io/webappsec-csp/#contains-a-header-delivered-content-security-policy">contains a header-delivered Content Security Policy</dfn> property.</li>
      <li>The <dfn data-x="parse-response-csp" data-x-href="https://w3c.github.io/webappsec-csp/#parse-response-csp">Parse a response's Content Security Policies</dfn> algorithm.</li>
@@ -11710,6 +11712,15 @@ partial interface <dfn id="document" data-lt="">Document</dfn> {
   the <span>ongoing navigation</span>. This eventually gets set back to null, after <cite>WebDriver
   BiDi</cite> considers the loading process to be finished. <ref>BIDI</ref></p>
 
+  <p>Each <code>Document</code> has a boolean
+  <dfn export data-x="concept-document-responsive-embedded-sizing">responsive embedded
+  sizing</dfn>, initially false.</p>
+
+  <p class="note">This is set by <code>meta</code> elements whose
+  <code data-x="attr-meta-name">name</code> attribute is
+  <code data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code>,
+  and used by <span>responsively-sized iframes</span>.</p>
+
   <p>Each <code>Document</code> has an <dfn data-x="concept-document-about-base-url">about base
   URL</dfn>, which is a <span>URL</span> or null, initially null.</p>
 
@@ -17782,6 +17793,10 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
   specified, then the <code data-x="attr-meta-content">content</code> attribute must also be
   specified. Otherwise, it must be omitted.</p>
 
+  <p>The <dfn element-attr for="meta"><code data-x="attr-meta-allowed-origins">allowed-origins</code></dfn>
+  attribute specifies the <span>ancestor-source-list</span>
+  for the <code data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code>.
+
   <p>The <dfn element-attr for="meta"><code data-x="attr-meta-charset">charset</code></dfn>
   attribute specifies the <span data-x="encoding">character encoding</span> used by the document.
   This is a <span>character encoding declaration</span>. If the attribute is present, its value must
@@ -18056,6 +18071,46 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
     </div>
    </dd>
 
+   <dt><dfn attr-value for="meta/name"><code data-x="meta-responsive-embedded-sizing">responsive-embedded-sizing</code></dfn></dt>
+
+   <dd>
+    <p>The element must have an <code
+    data-x="attr-meta-allowed-origins">allowed-origins</code> attribute,
+    and that attribute's value must be an <span>ancestor-source-list</span>.</p>
+
+    <div algorithm>
+    <p>If any <code>meta</code> element <var>element</var> is <span data-x="node is inserted into a
+    document">inserted into the document</span>, or has its <code
+    data-x="attr-meta-name">name</code> or <code data-x="attr-meta-content">content</code>
+    attributes changed, user agents must run the following algorithm:</p>
+
+    <ol>
+     <li><p>If <var>element</var> is not <span>in a document tree</span>, then return.</p></li>
+
+     <li><p>If <var>element</var> does not have 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-responsive-embedded-sizing">responsive-embedded-sizing</code>",
+     then return.</p></li>
+
+     <li><p>If <span>the body element</span> of the <code>Document</code> is not null,
+     then return.</p></li>
+
+     <li><p>If <var>element</var> does not have an <code
+     data-x="attr-meta-allowed-origins">allowed-origins</code> attribute,
+     or that attribute's value is not an <span>ancestor-source-list</span>,
+     or that attribute's value does not allow the <span
+     data-x="dom-hyperlink-origin">origin</span> of the <span
+     data-x="doc-container-document">container document</span>
+     by the <span>ancestor-source-list</span> syntax,
+     then return.</p></li>
+
+     <li><p>Set the <code>Document</code>'s
+     <span data-x="concept-document-responsive-embedded-sizing">responsive embedded sizing</span>
+     to true.<p></li>
+    </ol>
+    </div>
+   </dd>
+
    <dt><dfn attr-value for="meta/name"><code data-x="meta-theme-color">theme-color</code></dfn></dt>
 
    <dd>