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chromium
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chromium/chrome/test/data/tooltip_in_iframe.html 

<html>
<head></head>
<body style="margin: 0;">
  <button title="my tooltip">My button</button>
  <br>
  <iframe id="iframe1" src="tooltip.html" marginwidth="0" marginheight="0" frameborder="0" />
</body>
</html>