chromium/third_party/blink/web_tests/external/wpt/fedcm/support/fencedframe-mark-signedin.html

<!DOCTYPE html>
<title>A page that uses fencedframe to set the login status to signed in</title>

<fencedframe></fencedframe>
<script>
// If this page was opened as a popup, notify the opener when we are done loading.
if (window.opener) {
  window.onload = function() {
    window.opener.postMessage("done_loading", "*");
  };
}

// This code is intentionally after the onload listener registration
// because it can throw if FencedFrameConfig is not defined. In that
// case, we still want to notify the opener to avoid a test timeout.
const url = new URL("mark_signedin", location.href);
document.querySelector("fencedframe").config = new FencedFrameConfig(url);

</script>