chromium/third_party/blink/web_tests/external/wpt/appmanifest/id-member/id-member-service-worker.js

// Some user agents only offer app installation if there is a SW and it handles
// offline requests.

self.addEventListener("fetch", e => {
  e.respondWith(fetch(e.request).catch(_ => {
    return new Response('Offline test.');
  }));
});