The rendering calls are basically just passed to the hardware and WebGL is based on the OpenGL ES 2.0 specifications, which is really nice and allows for all common rendering techniques and around the same speed as it would have outside the browser.
The big problem however is, as ventilator already pointed out the javascript performance. Too many calls to the WebGL API will cause the gpu to wait for the cpu, which is a common problem, but will be much worse with javascript compared to native compiled code. Also already basic gameplay will be quite slow if you don´t carefully optimize everything. Then there are the browser differences: when it runs at acceptable speed in chrome, it will probably be unplayable in firefox, as javascript in chrome is a lot faster (but still nowhere close to native code).

WebGL works in most Android browsers (with extremely slow javascript), on Linux, Windows and OSX. It also works in iAds on iOS, but so far is not supported in Safari on iOS. WebGL is only really supported in Chrome and Firefox, while Internet Explorer doesn´t and the others have it in some kind of experimental state or something.