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chromium/content/test/data/accessibility/aria/aria-listbox-activedescendant.html 

<!--
@EXECUTE-AND-WAIT-FOR:focusChildren()
@MAC-ALLOW:AXOrientation
@MAC-ALLOW:AXSelected
@MAC-ALLOW:AXSelectedChildren
@MAC-ALLOW:AXVisibleChildren
-->
<html>
<body>
<div role="listbox" tabIndex="0" aria-activedescendant="3">
  <div id="1" aria-label="1" role="option">Item 1</div>
  <div id="2" aria-label="2" role="option">Item 2</div>
  <div id="3" aria-label="3" role="option">Item 3</div>
</div>
<div id="done"></div>
<script>
function focusChildren() {
    document.querySelector('*[role="listbox"]').focus();
    document.getElementById('done').innerText = 'focused';
    return 'focused';
}
</script>

</body>
</html>