Specify document conformance for headingoffset/headingreset

Also add to the attribute index. Fixes #11977.

Author: zcorpan

source

@@ -20243,9 +20243,12 @@ interface <dfn interface>HTMLHeadingElement</dfn> : <span>HTMLElement</span> {
   <p>The <dfn element-attr for="global"><code data-x="attr-headingoffset">headingoffset</code></dfn>
   content attribute allows authors to offset heading levels for descendants.</p>
 
+  <p>If the <code data-x="attr-headingoffset">headingoffset</code> attribute is specified, it must
+  have a value that is a <span>valid non-negative integer</span> between 0 and 8, inclusive.</p>
+
   <p>The <dfn element-attr for="global"><code data-x="attr-headingreset">headingreset</code></dfn>
-  content attribute allows authors to prevent a heading offset computation from traversing beyond
-  the element with the attribute.</p>
+  content attribute is a <span>boolean attribute</span>. It allows authors to prevent a heading
+  offset computation from traversing beyond the element with the attribute.</p>
 
   <div algorithm>
   <p>To <dfn>get an element's computed heading level</dfn>, given an element
@@ -153029,6 +153032,16 @@ interface <dfn interface>External</dfn> {
           <code data-x="attr-tdth-headers">th</code>
      <td> The header cells for this cell
      <td> <span>Unordered set of unique space-separated tokens</span> consisting of IDs*
+    <tr>
+     <th> <code data-x="">headingoffset</code>
+     <td> <span data-x="attr-tabindex">HTML elements</span>
+     <td> Offsets heading levels for descendants
+     <td> <span>Valid non-negative integer</span> between 0 and 8
+    <tr>
+     <th> <code data-x="">headingreset</code>
+     <td> <span data-x="attr-tabindex">HTML elements</span>
+     <td> Prevents a heading offset computation from traversing beyond the element with the attribute
+     <td> <span>Boolean attribute</span>
     <tr>
      <th> <code data-x="">height</code>
      <td> <code data-x="attr-canvas-height">canvas</code>;