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chromium/third_party/blink/web_tests/external/wpt/css/css-anchor-position/position-try-backdrop.html 

<!DOCTYPE html>
<title>CSS Anchor Positioning: position-try-fallbacks on ::backdrop</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-position-1/#fallback-apply">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  @position-try --pt {
    left: 300px;
  }
  #anchor {
    width: 100px;
    height: 100px;
    margin-left: 150px;
    margin-top: 50px;
    background: coral;
  }
  dialog::backdrop {
    background: seagreen;
    width:100px;
    height:100px;
    left: 9999px; /* Force overflow */
    position-try-fallbacks: --pt;
  }
</style>
<div id=anchor>Anchor</div>
<dialog id=dialog>Dialog</dialog>
<script>
  test((t) => {
    dialog.showModal();
    let style = getComputedStyle(dialog, '::backdrop');
    assert_equals(style.left, '300px');
  }, '::backdrop can use position-try-fallbacks');
</script>