allow multiple main elements when only one is visible

fixes #761

sections/semantics-grouping-content.include

@@ -1641,7 +1641,25 @@
   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 <{main}> in a document.
+  There must not be more than one visible <{main}> element in a document. If greater than one 
+  <{main}> element is present in a document, all other instances must be hidden using the <{hidden}> attribute.
+
+  <div class="example">
+  <pre highlight="html">
+  <!-- conforming example -->
+  <main></main>
+  <main hidden></main>
+  <main hidden></main>
+
+  <!-- non-conforming example -->
+  <main></main>
+  ...
+  <main></main>
+  ...
+  <main></main>
+  </pre>
+  </div>
+
 
   Authors must not include the <{main}> element as a descendant of an <{article}>,
   <{aside}>, <{footer}>, <code>header</code> or <{nav}> element.