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chromium/third_party/blink/web_tests/external/wpt/permissions-policy/resources/nested-sandbox.html 

<!DOCTYPE html>
<title>Return fullscreen permissions policy state</title>
<script>
  "use strict";
  window.onload = () => {
    window.parent.postMessage(document.featurePolicy.allowedFeatures().includes("fullscreen"),"*");
  };
</script>