Fix bugs in source parser

Fixes #9648, fixes #9649

source

@@ -29769,9 +29769,8 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
      <p>This example is the same as the previous example, but the image is <span
      data-x="attr-loading-lazy-state">lazy-loaded</span>. In this case, the <code
      data-x="attr-img-sizes">sizes</code> attribute can use the <code
-     data-x="valdef-sizes-auto">auto</code> keyword (or the <code
-     data-x="attr-img-sizes">sizes</code> attribute can be omitted), and the user agent will use the
-     <code data-x="attr-dim-width">width</code> attribute (or the width specified in CSS) for the
+     data-x="valdef-sizes-auto">auto</code> keyword, and the user agent will use the <code
+     data-x="attr-dim-width">width</code> attribute (or the width specified in CSS) for the
      <span>source size</span>.</p>
 
 <pre><code class="html">&lt;img <strong>loading="lazy" width="200" height="200" sizes="auto"</strong>
@@ -30068,7 +30067,8 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
   <span>update the image data</span> or <span data-x="img-environment-changes">react to environment
   changes</span> algorithms) <span>allows auto-sizes</span> and is <span>being rendered</span>,
   then <code data-x="valdef-sizes-auto">auto</code> is the <span>concrete object size</span> width.
-  Otherwise, <code data-x="valdef-sizes-auto">auto</code> is <code data-x="">100vw</code>.</p>
+  Otherwise, the <code data-x="valdef-sizes-auto">auto</code> value is ignored and the next
+  <span>source size</span> is used instead, if any.</p>
 
   <p>The <code data-x="valdef-sizes-auto">auto</code> keyword may be specified in the <code
   data-x="attr-source-sizes">sizes</code> attribute of <code>source</code> elements and <code
@@ -31406,13 +31406,28 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
      Otherwise, there is a <span data-x="concept-microsyntax-parse-error">parse error</span>;
      <span>continue</span>.</p></li>
 
-     <li><p>Remove all consecutive <span>&lt;whitespace-token></span>s
-     from the end of <var>unparsed size</var>.
-     If <var>unparsed size</var> is now empty,
-     then return <var>size</var>.
-     If this was not the keyword <code data-x="valdef-sizes-auto">auto</code> and
-     it was not the last item in <var>unparsed sizes list</var>,
-     that is a <span data-x="concept-microsyntax-parse-error">parse error</span>.</p></li>
+     <li>
+      <p>If <var>size</var> is <code data-x="valdef-sizes-auto">auto</code>, and <var>img</var>
+      is not null, and <var>img</var> is <span>being rendered</span>, and <var>img</var>
+      <span>allows auto-sizes</span>, then set <var>size</var> to the <span>concrete object
+      size</span> width of <var>img</var>, in <span data-x="'px'">CSS pixels</span>.</p>
+
+      <p class="note">If <var>size</var> is still <code data-x="valdef-sizes-auto">auto</code>,
+      then it will be ignored.</p>
+     </li>
+
+     <li>
+      <p>Remove all consecutive <span>&lt;whitespace-token></span>s from the end of <var>unparsed
+      size</var>. If <var>unparsed size</var> is now empty:</p>
+
+      <ol>
+       <li><p>If this was not the last item in <var>unparsed sizes list</var>, that is a <span
+       data-x="concept-microsyntax-parse-error">parse error</span>.</p></li>
+
+       <li><p>If <var>size</var> is not <code data-x="valdef-sizes-auto">auto</code>, then return
+       <var>size</var>. Otherwise, continue.</p></li>
+      </ol>
+     </li>
 
      <li><p>Parse the remaining <span data-x="component value">component values</span> in <var>unparsed size</var>
      as a <span>&lt;media-condition></span>.
@@ -31421,18 +31436,11 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
      <span>continue</span>. <ref>MQ</ref></p></li>
 
      <li><p>If <var>size</var> is not <code data-x="valdef-sizes-auto">auto</code>, then return
-     <var>size</var>.</p></li>
+     <var>size</var>. Otherwise, continue.</p></li>
     </ol>
    </li>
 
-   <li><p>Assert: <var>size</var> is null or <code data-x="valdef-sizes-auto">auto</code>.</p></li>
-
-   <li><p>If <var>img</var> is null, or <var>img</var> is not <span>being rendered</span>, or
-   <var>img</var> does not <span data-x="allows auto-sizes">allow auto-sizes</span>, then return
-   <code data-x="">100vw</code>.</p></li>
-
-   <li><p>Return the <span>concrete object size</span> width of <var>img</var>, in <span
-   data-x="'px'">CSS pixels</span>.</p></li>
+   <li><p>Return <code data-x="">100vw</code>.</p></li>
   </ol>
 
   <p class="note">It is invalid to use a bare <span>&lt;source-size-value></span> that is a <span>&lt;length></span>