Currently there is only one version on WebGL and just a hand-full extensions. But this does not boil down the question of WebGL support to a simple yes/no question. For some requirements of the GPU, WebGL only defines a lower limit to support (e.g. tiny textures of just 64*64 texels!) but each implementation can support more than that (you can check your browsers support and limits with my WebGL Extension Viewer). So if this in not taken into account while developing a 3D web application on a nice, hi-end developer machine you could end up with an app that does not run on mobile devices! Just targeting the lowest denominator might also not work depending on what you want to realise.
Luckily Florian Boesch set up the site WebGLStats to gather WebGL support and implementation dependent limits and presents the data in nice diagrams. This way it gets easier to figure out the right tradeoff to target at.