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

<!DOCTYPE html>
<link rel="match" href="modal-dialog-backdrop-ref.html">
<link rel="help" href="https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults">
<style>
dialog {
    top: 100px;
    height: 100px;
    width: 100px;
    background: green;
    outline: none;
}
</style>
<body>
Test for the default user agent style of dialog::backdrop. The test passes if
there is a green box, above a very lightly translucent gray box spanning the
viewport.
<dialog></dialog>
<script>
document.querySelector('dialog').showModal();
</script>
</body>