The WebGLworking group has Google, Mozilla, Apple and Opera and finalized 1.0 back in March.
Microsoft has not implemented WebGL for IE9 and their strategy is unknown (there are plugin workarounds).
Because WebGL is based on OpenGL ES 2.0 it can be easily transitioned to the mobile web.
Competing/complementary technologies include Microsoft DirectX, OpenGL itself, Java(FX?), Flash and Silverlight.
A big plus for WebGL (and Canvas) is that, apart from being browser based, no plugin/download is required. WebGL graphics while not in the same class as DirectX and OpenGL, are quite good.