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chromium/content/test/data/accessibility/html/details.html 

<!--
@MAC-ALLOW:AXRoleDescription
@MAC-ALLOW:AXExpanded=*
@UIA-WIN-ALLOW:LocalizedControlType='details'
@WIN-ALLOW:EXPANDED
@WIN-ALLOW:COLLAPSED
@WIN-ALLOW:localized_extended_role=*
@AURALINUX-ALLOW:expand*
@AURALINUX-ALLOW:focus*
-->
<!DOCTYPE html>
<html>
<body>
  <details>
    <summary>details tag</summary>
    <p>The details tag specifies additional details that the user can view or hide on demand.</p>
  </details>

  <details open>
    <summary>details tag open</summary>
    <p>The details tag with open specifies that the details should be visible (open) to the user.</p>
  </details>
</body>
</html>