1. Clone the repo
2. Create your project on the Firebase Console.
3. Open Project and go to Project settings > Cloud Messaging and there in section Web configuration click Generate key pair button.
4. Copy public key and in index.html file replace <YOUR_PUBLIC_VAPID_KEY_HERE> with your key.
5. Copy your Firebase Config object in index.html (line 108) and firebase-messaging-sw.js (line 65) replacing the existing object.
6. Serve the folder locally using Python Simple HTTP Server (python -m SimpleHTTPServer 9000)
7. Open Chrome Debugging Tools, go to more tools, remote devices.
8. Connect your Android phone and enable USB debugging.
9. Once your phone is listed in the remote devices section, enable port forwarding for port 9000.
10. Open localhost:9000 on your phone and grant access to notifications.
11. Copy your FCM Token.
12. Obtain your server key from the project settings, cloud messaging tab.
13. Send a cURL request like this

curl -X POST \
  https://fcm.googleapis.com/fcm/send \
  -H 'Authorization: key=<LEGACY_SERVER_KEY_HERE>' \
  -H 'Content-Type: application/json' \
  -d '{
  "data": {
      "title": "FCM Message",
      "body": "This is a message from FCM"
  },
  "to": "<FCM_TOKEN_HERE>"
}'

14. Click on the notification received on your device. The notification will open up normally in Chrome.
15. Now add localhost:9000 to your Home Screen as PWA.
16. Send the same cURL request again.
17. Click on the notification.
18. Notification won't open up