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

<!doctype html>
<title>dialog: overlay</title>
<link rel="help" href="https://html.spec.whatwg.org/multipage/interactive-elements.html#the-dialog-element">
<link rel="help" href="https://drafts.csswg.org/css-position-4/#overlay">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<dialog id="dialog"></dialog>
<script>
  const dialog = document.getElementById("dialog");

  test(() => {
    assert_equals(getComputedStyle(dialog).overlay, "none",
                  "Computed overlay");
    assert_equals(getComputedStyle(dialog, "::backdrop").overlay, "none",
                  "Computed overlay for ::backdrop");
  }, "dialog computed overlay initially 'none'");

  test(() => {
    dialog.showModal();

    assert_equals(getComputedStyle(dialog).overlay, "auto",
                  "Computed overlay on open dialog");
    // ::backdrop pseudo element is always rendered in the top layer when its
    // originating element is. It does not get its overlay property changed,
    // though.
    assert_equals(getComputedStyle(dialog, "::backdrop").overlay, "none",
                  "Computed overlay for ::backdrop");

    dialog.close();

    assert_equals(getComputedStyle(dialog).overlay, "none",
                  "Computed overlay on closed dialog");
    assert_equals(getComputedStyle(dialog, "::backdrop").overlay, "none",
                  "Computed overlay for ::backdrop");
  }, "Opening and closing a modal dialog changes computed overlay to 'auto' and back to 'none'");
</script>