Last time I looked at it the WebGL implementation was not complete. Hopefully the IE team switches to a monthly release cycle like Chrome and FF so we don't need to wait until Window8.2 for proper WebGL support. Stuff that didn't work: no support for glBindAttribLocation, non-trivial shaders didn't compile, and no extensions at all (so no compressed textures). Especially the missing compressed texture support is a deal breaker for games.