If history is any indication, it won't be WebGL but WebDX and we'll at least write both glsl and hlsl if not directx adapted to javascript outright.

They can alternatively just require at least opengl es 2.0 support installed through gfx driver, thus only working on desktop machines. Not that it will make their plans of phone+tablet+desktop unification easier but, hey, it might shut up whining hipster developers for at least a few years until the realization that MS intentionally underdelivers dawns on them.