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chromium/third_party/blink/web_tests/external/wpt/speculation-rules/prerender/resources/cross-origin-isolated-iframe.https.html 

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<body>
<script>
assert_true(document.prerendering);
parent.postMessage(
    {name: 'crossOriginIsolated', value: self.crossOriginIsolated}, '*');
</script>
</body>