Refines main element definition (#1154)

* Refines main element definition
* Simplifies the wording to align with the WHATWG
* Fixes #1153

* Restores second note

sections/semantics-grouping-content.include

@@ -1644,8 +1644,7 @@
     <dd><a>Flow content</a>.</dd>
     <dd><a>Palpable content</a>.</dd>
     <dt><a>Contexts in which this element can be used</a>:</dt>
-    <dd>Where <a>flow content</a> is expected, but with no <{article}>, <{aside}>, <{footer}>,
-    <{header}> or <{nav}> element ancestors.</dd>
+    <dd>Where <a>flow content</a> is expected, but only if it is a <dfn>hierarchically correct main element</dfn>.</dd>
     <dt><a>Content model</a>:</dt>
     <dd><a>Flow content</a>.</dd>
     <dt><a>Tag omission in text/html</a>:</dt>
@@ -1664,19 +1663,20 @@
     <dd>Uses <code>HTMLElement</code></dd>
   </dl>
 
-  The <{main}> element <a>represents</a> the main content of the <{body}> of a document or application.
+  The <{main}> element <a>represents</a> the dominant contents of the document.
+
+  A document must not have more than one <{main}> element that does not have the [[#the-hidden-attribute]] attribute specified.
+
+  A <a>hierarchically correct main element</a> is one whose ancestor elements are limited to <{html}>, <{body}>, <{div}>, <{form}> without an <a>accessible name</a>, and <a>autonomous custom elements</a>. Each <{main}> element must be a <a>hierarchically correct main element</a>.
 
   <p class="note">
     The <{main}> element is not <a>sectioning content</a> and has no effect on the document
     <a>outline</a>.
   </p>
-  The main <a>content area</a> of a document includes content that is unique to that document and excludes
-  content that is repeated  across a set of documents such as site navigation links, copyright
-  information, site logos and banners and search forms (unless the document or application's main
-  function is that of a search form).
 
-  There must not be more than one visible <{main}> element in a document. If more than one
-  <{main}> element is present in a document, all other instances must be hidden using [[#the-hidden-attribute]].
+  <p class="note">
+  The <{main}> element is not suitable for identifying the main <a>content areas</a> of sub-sections of a document. The simplest solution is not to mark up the main content of a sub-section at all (leaving it as implicit), but an author could use [[#grouping-content]] or a <a>sectioning content</a> element as appropriate.
+  </p>
 
   <div class="example">
   <pre highlight="html">
@@ -1692,17 +1692,6 @@
   </pre>
   </div>
 
-
-  Authors must not include the <{main}> element as a descendant of an <{article}>,
-  <{aside}>, <{footer}>, <{header}> or <{nav}> element.
-
-  <p class="note">
-    The <{main}> element is not suitable for use to identify the main <a>content areas</a> of sub
-    sections of a document or application. The simplest solution is to not mark up the main content
-    of a sub section at all, and just leave it as implicit, but an author could use a
-    [[#grouping-content]] or <a>sectioning content</a> element as appropriate.
-  </p>
-
     In the following example, we see 2 articles about skateboards (the main topic of a Web page) the
     main topic content is identified by the use of the <{main}> element.