So, to clarify, are you looking to make an HTML5/WebGL game engine (along with a proof of concept game)? Or something more like GameMaker, where people can design games using a simple drag-and-drop style interface without code, but web-based? Or are you just looking to make a basic proof-of-concept game that's open to expansion?
As far as game engines go, you should check out Babylon.js.
I'm sure there are good examples out there of something closer to GameMaker, but perhaps someone else could speak of those - I haven't tried any.