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chromium/content/test/data/accessibility/html/iframe-scrollable.html 

<!--
@BLINK-ALLOW:scrollable=*
-->
<!DOCTYPE html>
<html style="width:100px; height:100px;">
<body>
  <iframe style="width:200px; height: 200px;" aria-label="Scrollable iframe" src="frame/visible_text.html">
  </iframe>
</body>
</html>