I could see canvas extended to have 3D operations. Like say draw point takes 3 coordinates now. I can see a third party library that does 3D but projects it down to 2D and uses regular 2D commands on the canvas, or I see supporting WebGL interface and writing OpenGL style code.
I think that's the role WebGL essentially fulfills. It was just done in a way that uses OpenGL instead of reinventing the wheel, since a 3D rendering engine is a non-trivial thing to build. In other words, you can do either:
var ctx = myCanvas.getContext('2d');
// ctx has the 2d api
var ctx = myCanvas.getContext('webgl');
// ctx has the webgl 3d api
I think it would be a lot slower. Either way there's a 3D rendering engine. Embedding the engine natively and having GPU speedups is a dominating factor.
> or I see supporting WebGL interface and writing OpenGL style code.
See first point above.