>How do specs fit into this if every browser is just a UI shell around chrome?

The browser engine is open source. You can make changes and proposals with your own fork. If you are making a browser for musicians you could try and advance the WebMIDI spec and add new functionality to it and then try to get it standardized. But as a browser developer you don't want to me spending your time implenting specs that other people have already implemented, but rather spending time working on things that improve your browser for your userbase.