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chromium/third_party/blink/web_tests/http/tests/feature-policy/resources/blank.html 

<!DOCTYPE html>
<html>
<title>Informs the test runner when this page has loaded</title>
<script>
window.addEventListener('DOMContentLoaded', () => {
  if (window.testRunner)
    testRunner.notifyDone();
});
</script>
</html>