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chromium/content/test/data/accessibility/event/live-region-add.html 

<!DOCTYPE html>
<html>
<body>
<div id="live" aria-live="polite">Before</div>
<script>
  function go() {
    var add = document.createElement('p');
    add.innerText = 'After';
    document.getElementById('live').appendChild(add);
  }
</script>
</body>
</html>