Push Notification vs Web Notification APIs, Can we push notifications to desktop browsers?

Answering your question point by point:

1. Yes you can use GCM to deliver notifications to Chrome Browser and Firefox as well. The Notification APIs were deprecated and new concept of service worker has evolved from Chrome 42. Check Link: Push Notifications on the Open Web | Web Updates - Google Developers

2. Not applicable

3. Check the link shared in 1. It has link to the GIT. You can also check out the specs here: https://notifications.spec.whatw...

Safari uses APNS while UC Browser is also experimenting with Facebook to deliver notifications on Browsers. What browser is using what among GCM and APNS will depend on underlying OS i.e. for Android, browsers will have to use GCM and APNS for iOS.