chromium/third_party/blink/web_tests/external/wpt/partitioned-popins/resources/partitioned-popins.localStorage-window.html

<!doctype html>
<meta charset="utf-8">
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<script>
(async function() {
  test_driver.set_test_context(window.opener);

  // Step 3 (partitioned-popins/partitioned-popins.localStorage.tentative.sub.https.window.js)
  window.localStorage.setItem("first-party", (new URLSearchParams(window.location.search)).get("id"));
  window.opener.postMessage({type: "window-set", message: "Set first-party data"}, "*");
  window.close();
})();
</script>