gecko-dev/dom/manifest/test/file_reg_appinstalled_event.html

<meta charset=utf-8>
<script>
"use strict";
window.addEventListener("appinstalled", () => {
  document
    .querySelector("#output")
    .innerHTML = "event received!";
  // Send a custom event back to the browser
  // to acknowledge that we got this
  const detail = { result: true }
  const ev = new CustomEvent("dom.manifest.onappinstalled", { detail });
  document.dispatchEvent(ev);
});
</script>
<h1 id=output>waiting for event</h1>