Editorial: hgroup makes multi-level heading in outline

This change clarifies the fact that the outline algorithm requires
implementations to make any hgroup element in the source result in a
multi-level heading in the generated outline.

source

@@ -15891,40 +15891,55 @@ Space is not the only void</pre>
    <p>The following markup fragment:</p>
 
    <pre>&lt;body>
- &lt;h1>A&lt;/h1>
- &lt;p>B&lt;/p>
- &lt;h2>C&lt;/h2>
- &lt;p>D&lt;/p>
- &lt;h2>E&lt;/h2>
- &lt;p>F&lt;/p>
+  &lt;hgroup id="document-title">
+    &lt;h1>HTML&lt;/h1>
+    &lt;h2>Living Standard — Last Updated 12 August 2016&lt;/h2>
+  &lt;/hgroup>
+  &lt;p>Some intro to the document.&lt;/p>
+  &lt;h2>Table of contents&lt;/h2>
+  &lt;ol id=toc>...&lt;/ol>
+  &lt;h2>First section&lt;/h2>
+  &lt;p>Some intro to the first section.&lt;/p>
 &lt;/body></pre>
 
    <p>...results in the following outline being created for the <code>body</code> node (and thus the
    entire document):</p>
 
    <ol class="brief">
     <li>
-     <p>Section created for <code>body</code> node.</p>
-     <p>Associated with heading "A".</p>
-     <p>Also associated with paragraph "B".</p>
+     <p><strong>Section created for <code>body</code> node</strong>.</p>
+     <p>Associated with heading <code data-x="">&lt;hgroup
+     id="document-title">...&lt;/hgroup></code> consisting of primary heading <code
+     data-x="">&lt;h1>HTML&lt;/h1></code> and secondary heading <code data-x="">&lt;h2>Living
+     Standard — Last Updated 12 August 2016&lt;/h2></code>.</p>
+     <p>Also associated with the paragraph <code data-x="">&lt;p>Some intro to the
+     document.&lt;/p></code> (though it likely would not be shown in a rendered view of the
+     outline).</p>
      <p>Nested sections:</p>
      <ol class="brief">
       <li>
-       <p>Section implied for first <code>h2</code> element.</p>
-       <p>Associated with heading "C".</p>
-       <p>Also associated with paragraph "D".</p>
+       <p><strong>Section implied for first <code>h2</code> element.</strong></p>
+       <p>Associated with heading <code data-x="">&lt;h2>Table of contents&lt;/h2></code>.</p>
+       <p>Also associated with the ordered list <code data-x="">&lt;ol id=toc>...&lt;/ol></code>
+       (though it likely would not be shown in a rendered view of the outline).</p>
        <p>No nested sections.</p>
       </li>
       <li>
-       <p>Section implied for second <code>h2</code> element.</p>
-       <p>Associated with heading "E".</p>
-       <p>Also associated with paragraph "F".</p>
+       <p><strong>Section implied for second <code>h2</code> element.</strong></p>
+       <p>Associated with heading <code data-x="">&lt;h2>First section>&lt;/h2></code>.</p>
+       <p>Also associated with the paragraph <code data-x="">&lt;p>Some intro to the first
+       section.&lt;/p></code> (though it likely would not be shown in a rendered view of the
+       outline).</p>
        <p>No nested sections.</p>
       </li>
      </ol>
     </li>
    </ol>
 
+   <p>The following image shows what a rendered view of the outline might look like.</p>
+
+   <p><img src="/images/outline.svg" width="465" height="120"
+           alt="Outline showing rendering of a multi-level heading and other headings."></p>
   </div>
 
   <div w-nodev>
@@ -16124,6 +16139,14 @@ Space is not the only void</pre>
       <p>If the <var>current section</var> has no heading, let the element being entered be
       the heading for the <var>current section</var>.</p>
 
+      <p class="note">If the element being entered is an <code>hgroup</code> element, that
+      <code>hgroup</code> as a whole is a <em>multi-level</em> heading for the <var>current
+      section</var>, with the highest-<span data-x="rank">ranked</span>
+      <code>h1</code>&ndash;<code>h6</code> descendant of the <code>hgroup</code> providing the
+      primary heading for the <var>current section</var>, and with other
+      <code>h1</code>&ndash;<code>h6</code> descendants of the <code>hgroup</code> providing
+      secondary headings for the <var>current section</var>.</p>
+
       <p>Otherwise, if the element being entered has a <span>rank</span> equal to or higher than the
       heading of the last section of the <span>outline</span> of the <var>current outline
       target</var>, or if the heading of the last section of the <span>outline</span> of the <var>current outline target</var> is an implied heading, then create a new <span