fixes #9217 manually--use main element instead of div in home and default layouts

_layouts/default.html

@@ -42,9 +42,9 @@
             </ul>
           </div>
         </div>
-        <div class="col-lg-9" role="main">
+        <main class="col-lg-9" role="main">
           {{ content }}
-        </div>
+        </main>
       </div>
 
     </div>

index.html

@@ -4,7 +4,7 @@ title: Bootstrap
 base_url: "./"
 ---
 
-<div class="bs-masthead" id="content" role="main">
+<main class="bs-masthead" id="content" role="main">
   <div class="container">
     <h1>Bootstrap 3</h1>
     <p class="lead">Sleek, intuitive, and powerful mobile-first front-end framework for faster and easier web development.</p>
@@ -13,4 +13,4 @@ base_url: "./"
     </p>
     <p><strong>Heads up!</strong> Downloads are pulled directly from the latest commited code on GitHub, and as a result our docs may at times be out of sync.</p>
   </div>
-</div>
+</main>