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chromium/third_party/blink/web_tests/external/wpt/fullscreen/api/resources/echo-fullscreenEnabled.html 

<!doctype html>
<script>
window.onmessage = (e) => {
  if (e.data === 'What is document.fullscreenEnabled?') {
    e.source.postMessage(document.fullscreenEnabled, '*');
  } else {
    e.source.postMessage('Incorrect message', '*');
  }
}
</script>
<p>This page echos document.fullscreenEnabled.</p>