chromium/third_party/blink/web_tests/external/wpt/fetch/private-network-access/resources/fetcher.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>Fetcher</title>
<script>
  window.addEventListener("message", function (event) {
    const { url, options } = event.data;
    fetch(url, options)
        .then(async function(response) {
          const body = await response.text();
          const message = {
            ok: response.ok,
            type: response.type,
            body: body,
          };
          parent.postMessage(message, "*");
        })
        .catch(error => {
          parent.postMessage({ error: error.toString() }, "*");
        });
  });
</script>