Hm, i would have to disappoint you then, noone has written such a polyfill yet, because you can't recreate the native API with javascript. It's like trying to recreate WebGL, WebRTC or WebSockets in Javascript, which isn't possible, you need native Support for in somehow in your Browser.

In the Case of HTML5 Notifications you would have to rewrite it in HTML5, CSS3 and Javascript to show them on a Website, but you can't show them on your Desktop then.
So what i was trying to do with this PR was to detect if you somehow have native Methods already in your Browser to show Desktop Notifications or if you have to "Fallback" to Notifications on your Website.