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chromium/third_party/blink/web_tests/shadow-dom/imperative-apis/test-custom-detail-summary.html 

<!DOCTYPE html>
<script src="custom-detail-summary.js"></script>

<my-detail id="my-detail">
  <p>another test</p>
  <my-summary id="my-summary">summary</my-summary>
</my-detail>
<my-summary id="my-summary1">added-summary</my-summary>
<script>
const host = document.querySelector("#my-detail");
const sum = document.querySelector("#my-summary");
const sum1 = document.querySelector("#my-summary1");
host.insertBefore(sum1, sum);
</script>