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chromium/third_party/blink/web_tests/external/wpt/selection/shadow-dom/cross-shadow-boundary-3-ref.html 

<!doctype html>
OuterText
<div id="host1">innerText1</div>
OuterText
<div id="host2">innerText2</div>
<script>
  const host1 = document.getElementById("host1");
  const host2 = document.getElementById("host2");

  getSelection().setBaseAndExtent(
    host1.firstChild, 3, host2.firstChild, 3);
</script>