chromium/content/test/data/accessibility/html/popover-expanded.html

<!--
@BLINK-ALLOW:expanded*
@WAIT-FOR:PopoverIsExpanded
-->

<!DOCTYPE html>

<button id=toggle popovertarget="will-expand">Will expand</button>
<div popover id="will-expand">PopoverIsExpanded</div>

<script>
document.addEventListener('DOMContentLoaded', () => {
  document.getElementById('toggle').click();
});
</script>