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chromium/content/test/data/accessibility/event/aria-setsize-changed.html 

<!DOCTYPE html>
<html>
<body>
  <ul role="listbox">
    <li id="li1" role="option">Li1</li>
    <li id="li2" role="option">Li2</li>
    <li id="li3" role="option" aria-setsize="5">Li3</li>
  </ul>>
  <script>
    function go() {
      // Set aria-setsize to '4'; should fire an event.
      document.getElementById('li1').setAttribute('aria-setsize', '4');

      // Set aria-setsize to -1'; should fire an event.
      document.getElementById('li2').setAttribute('aria-setsize', '-1');

      // Set aria-setsize from '5'->[removed]; should fire an event.
      document.getElementById('li3').removeAttribute('aria-setsize');
    }
  </script>
</body>
</html>