Cross-origin principal indicator

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<title>Cross-origin principal indicator</title>
<style>
  body { font: 18px system-ui; max-width: 40rem; margin: 4rem auto; padding: 1rem; }
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<h1>Cross-origin principal indicator</h1>
<p>This page exposes one benign test callback that ignores its argument.</p>
<script>
  window.proof = function proof() {
    alert(document.domain);
  };
</script>