<html>
<head>
<script>
var doMutation = function() {
  var region = document.getElementById("to_mutate");
  var new_child = document.createElement("div");
  new_child.appendChild(document.createTextNode("New child text contents"));
  region.appendChild(new_child);
  window.setTimeout(function() { window.close(); } );
}
window.addEventListener("load", doMutation);
</script>
</head>
<body>
<div id="to_mutate" aria-live="polite" aria-relevant="additions" aria-atomic="true">
   Original contents.
</div>
</body>
</html>