Why attempt something that has abundant number of libraries to pick and choose? To me, however impressive it is, 'browser build from scratch' simply overstates it. Why not attempt something like a 3D game where it's hard to find open source code to use?

Assets are very hard to produce and largely unsolved by AI at the moment.

Is something like a 3D game engine even hard to find source code for? There's gotta lots of examples/implementations scattered around.