chromium/third_party/blink/web_tests/http/tests/loading/sxg/resources/sxg-usecounter.html

<!DOCTYPE html>
<title>Content of SignedHTTPExchange</title>
<script>
const kSignedExchangeInnerResponse = 2541; // see web_feature.mojom

window.addEventListener('message', (event) => {
  const msg = {location: document.location.href, is_fallback: false};
  if (window.internals) {
    msg.is_use_counted = internals.isUseCounted(document, kSignedExchangeInnerResponse);
  }

  event.data.port.postMessage(msg);
}, false);
</script>