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chromium/content/test/data/accessibility/aria/aria-alert.html 

<!DOCTYPE html>
<!--
@MAC-ALLOW:AXRoleDescription='alert'
@MAC-ALLOW:AXARIAAtomic=*
@WIN-ALLOW:atomic:*
@WIN-ALLOW:container-atomic:*
@WIN-ALLOW:live:*
@BLINK-ALLOW:liveAtomic=true
@BLINK-ALLOW:containerLiveAtomic=true
@BLINK-ALLOW:liveStatus=*
-->
<html>
<body>
  <p id="ariaalert" role="alert"> This test is for aria role="alert" </p>
</body>
</html>