have warning support all browser versions

common/badBrowserCheckB.js

@@ -23,7 +23,7 @@ try {
 }
 
 if (badBrowser) {
-  var el = document.getElementsByClassName('BadBrowser')[0];
+  var el = document.getElementById('BadBrowserWarning');
   el.className += ' is-open';
   // Dumb check for known mobile OS's. Not important to catch all, just
   // displays more appropriate information.

common/index.html

@@ -37,7 +37,7 @@
     </div>
   </noscript>
 
-  <div class="BadBrowser" style="display: none;">
+  <div id="BadBrowserWarning" class="BadBrowser" style="display: none;">
     <div class="BadBrowser-content">
       <h2>Your Browser is Out of Date</h2>
       <p class="is-desktop">