WebGL support is improving over time, but the situation right now is:
Android: Firefox supports WebGL, but due to a bug (which will be fixed) it's pretty slow. Chrome beta supports enabling WebGL via chrome://flags, which in my testing works great, very promising. Hopefully will be eventually enabled by default in the stable Chrome.
iOS: supports WebGL, but is disabled. Hopefully Apple will eventually enable it.
Firefox OS and BB10 both have good support for WebGL as well, I believe. Windows Phone 8 doesn't and may not for a long time, because MS are against WebGL (it competes with DirectX).