sw.js

self.addEventListener('fetch', event => {
  let request = event.request;

  if (request.method == 'GET') {
    request = new Request(request.url, {
      method: 'GET',
      headers: request.headers,
      mode: request.mode == 'navigate' ? 'cors' : request.mode,
      credentials: request.credentials,
      redirect: request.redirect
    });
  }

  // now we don't hit the FAIL after a POST redirect!
  event.respondWith(
    fetch(request).catch(() => new Response("FAIL"))
  );
});