Three dimensional real-time graphics is very sophisticated at this point and much of its sophistication is due to the GPU (Graphics board) where all of the visual images are processed at enormous speeds. The trick is for webGL to take advantage of the Graphics Board's speed.
I am just beginning to experiment with some of the libraries based on webGL. If you are using Chrome or a webGL compatible (not IE) browser, you can see what I am doing right now (September 2013).
My first program using THREE.js: click here
A home video using THREE.js: click here
Experimenting with animation in THREE.js: click here
textured heads in THREE.js: click here
Try them out and you will see the potential for the browser to handle graphics. Although it is not yet fast enough to do very complex CG scenes, it is really far further along than I'd anticipated.