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chromium/third_party/blink/web_tests/external/wpt/selection/shadow-dom/cross-shadow-boundary-select-document-ref.html 

<!doctype html>
<html>
  <div>CONTENT</div>
  <script>
    const div = document.querySelector("div");
    getSelection().setBaseAndExtent(div.firstChild, 0, div.firstChild, 2);
  </script>
</html>