chromium/third_party/blink/web_tests/external/wpt/badging/resources/setAppBadge_iframe.html

<!DOCTYPE html>
<meta charset="utf-8" />
<title>setAppBadge iframe</title>
<script>
  async function callSetAppBadge() {
    const postMessageData = { message: "callSetAppBadge" };

    try {
      await navigator.setAppBadge();
      postMessageData.status = "success";
    } catch (e) {
      if (e instanceof DOMException) {
        postMessageData.status = "error";
        postMessageData.exceptionType = e.name;
      } else {
        postMessageData.status = "unknown_error";
        postMessageData.error = e.toString();
      }
    } finally {
      window.parent.postMessage(postMessageData, "*");
    }
  }

  window.addEventListener("message", async (event) => {
    switch (event.data) {
      case "callSetAppBadge":
        await callSetAppBadge();
        break;
      default:
        throw new Error(`Unexpected message: ${event.data}`);
    }
  });
</script>