chromium/third_party/blink/web_tests/html/details_summary/details-clone.html

<html>
<style>
details {
  display: inline-block;
  background: #eee;
  margin: 8px;
}
</style>
<body>
<p>Test to clone details and summary elements.  &lt;details> elements in the
second row should be same as elements in the first row.</p>

<hr>
<div class="container">
<details open>
   <summary>Summary 1</summary>
   Content 1
</details>
<details open>
   Content 2
</details>
<details>
   <summary>Summary 3</summary>
   Content 3
</details>
<details>
   Content 4
</details>
</div>
<hr>
<script>
document.body.appendChild(document.querySelector('.container').cloneNode(true));
</script>
</body>
</html>