Codebase Browser
chromium
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chromium/chrome/test/data/page_load_metrics/iframes.html 

<html>
  <link rel="stylesheet" href="seamless-iframe.css">
  <body>
    <iframe src="/title1.html" class="seamless"></iframe>
    <iframe src="/title1.html" class="seamless"></iframe>
  </body>
</html>