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chromium/chrome/test/data/page_load_metrics/react3_page.html 

<!DOCTYPE html>

<body>
  <div id="nothing"></div>
  <div id="container">
    <div data-reactroot="test">
      This is a test to see if a page with a div with data-reactroot attribute is
      detected as using the React JavaScript framework.
    </div>
  </div>
</body>