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chromium/third_party/blink/web_tests/external/wpt/html/semantics/interactive-elements/the-dialog-element/backdrop-inherits.html 

<!DOCTYPE html>
<link rel="match" href="backdrop-inherits-ref.html">
<link rel="help" href="https://drafts.csswg.org/css-position-4/#backdrop">
<style>
dialog {
  --backdrop-bg: green;
  visibility: hidden;
}

dialog::backdrop {
  position: absolute;
  top: 100px;
  left: 100px;
  height: 100px;
  width: 100px;
  visibility: visible;
  background-color: var(--backdrop-bg);
}
</style>
Test that ::backdrop inherits from its originating element. The test passes if
there is a green box and no red.
<dialog></dialog>
<script>
document.querySelector('dialog').showModal();
</script>