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chromium/third_party/blink/web_tests/external/wpt/html/semantics/interactive-elements/the-details-element/details-add-summary.html 

<!DOCTYPE html>
<html class="reftest-wait">
<link rel=match href="details-add-summary-ref.html">
<link rel=author href="mailto:[email protected]">
<link rel=help href="https://github.com/whatwg/html/pull/6466">

<!-- This test makes sure that new <summary> elements get rendered correctly
  when added to a <details> element. I ran into it when adding
  content-visibility:hidden to the second slot of <details>. -->

<script>
onload = () => {
  const newsummary = document.createElement('summary');
  newsummary.textContent = 'new summary';
  document.getElementById('detailsid').insertBefore(newsummary,
    document.getElementById('oldsummary'));

  document.documentElement.classList.remove('reftest-wait');
};
</script>

<details id=detailsid>
  <summary id=oldsummary>old summary</summary>
  details
</details>