How does Facebook send notifications without tab

Question

1. Go to facebook and allow HTML5 Desktop Notification for it.
2. Close facebook tab.
3. Ask somebody to write you message on facebook.

Now u s

Answer 1

They use the Push API - https://developer.mozilla.org/en-US/docs/Web/API/Push_API Quoting from there:

The Push API gives web applications the ability to receive messages pushed to them from a server, whether or not the web app is in the
foreground, or even currently loaded, on a user agent.