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chromium/third_party/blink/web_tests/external/wpt/html/infrastructure/safe-passing-of-structured-data/resources/iframe-resizable-arraybuffer-helper.html 

<!DOCTYPE html>
<script>

window.addEventListener('message', (e) => {
  const buffer = e.data;
  e.source.postMessage(`byteLength=${buffer.byteLength},maxByteLength=${buffer.maxByteLength},resizable=${buffer.resizable}`, '*');
});

window.parent.postMessage('started', '*');

</script>