<html>
<head>
<script>
var doMutation = function() {
  var region = document.getElementById("to_mutate");
  region.firstChild.textContent = "New text contents";

  window.setTimeout(function() { window.close(); } );
}
window.addEventListener("load", doMutation);
</script>
</head>
<body>
<div id="to_mutate" aria-live="polite" aria-relevant="text" aria-atomic="false">
   Original contents.
</div>
</body>
</html>