chromium/chrome/test/data/page_load_metrics/react6_page.html

<!DOCTYPE html>

<body>
  <div id="nothing"></div>
  <div id="container">
    <div id="target">
      This is a test to see if a page with a div with a property starting with
      __reactFiber is detected as using the React JavaScript framework.
    </div>
  </div>
  <script>
    document.getElementById("target").__reactFiber123test = "test";
  </script>
</body>