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

<!doctype html>
<head>
<link rel="match" href="cross-shadow-boundary-3-ref.html" />
</head>
OuterText
<div id="host1"></div>
OuterText
<div id="host2"></div>
<script>
  const root1 = document.getElementById("host1").attachShadow({ mode: "open" });
  root1.innerHTML = "innerText1";

  const root2 = document.getElementById("host2").attachShadow({ mode: "open" });
  root2.innerHTML = "innerText2";

  getSelection().setBaseAndExtent(root1.firstChild, 3, root2.firstChild, 3);
</script>