updated header/footer scoping to reflect acc semantics

Fixes: #520 thanks to @jasonkiss

sections/semantics-sections.include

@@ -959,12 +959,13 @@
     <dd>Uses {{HTMLElement}}.</dd>
   </dl>
 
-  The <{header}> element <a>represents</a> introductory content for its nearest ancestor
-  <a>sectioning content</a> or <a>sectioning root</a> element. A <code>header</code> typically
+  The <{header}> element <a>represents</a> introductory content for its nearest ancestor <{main}> 
+  element <a>sectioning content</a> or <a>sectioning root</a> element. A <code>header</code> typically
   contains a group of introductory or navigational aids.
 
   When the nearest ancestor <a>sectioning content</a> or <a>sectioning root</a> element is
-  <a href="#the-body-element">the <code>body</code> element</a>, then it applies to the whole page.
+  <a href="#the-body-element">the <code>body</code> element</a>, then it applies to the whole page, 
+   unless they are descendants of the <{main}> element.
 
   <p class="note">
     A <{header}> element is intended to usually contain the section's heading (an
@@ -1109,7 +1110,7 @@
   </dl>
 
   The <{footer}> element <a>represents</a> a footer for its nearest ancestor
-  <a>sectioning content</a> or <a>sectioning root</a> element. A footer typically contains
+  <{main}> element or <a>sectioning content</a> or <a>sectioning root</a> element. A footer typically contains
   information about its section such as who wrote it, links to related documents, copyright data,
   and the like.
 
@@ -1134,7 +1135,9 @@
 
   Footers don't necessarily have to appear at the <em>end</em> of a section, though they usually do.
 
-  When the nearest ancestor <a>sectioning content</a> or <a>sectioning root</a> element is <a href="#the-body-element">the <code>body</code> element</a>, then it applies to the whole page.
+  When the nearest ancestor <a>sectioning content</a> or <a>sectioning root</a> element is 
+  <a href="#the-body-element">the <code>body</code> element</a>, then it applies to the whole page, 
+   unless they are descendants of the <{main}> element.
 
   <p class="note">
     The <{footer}> element is not <a>sectioning content</a>; it doesn't introduce a new