Several new web technologies allow the graphics processing unit to be harnessed by the browser.
Flash the de-facto plugin standard from Adobe adds the necessary DrawTriangles and SetShader commands to it's Action Script 3 controlled Flash player 10.
Google have open-sourced and currently maintain a beta release of O3D. Originally the engine behind a failed second life clone, O3D stutters along in a a relatively convincing manner but as of yet failed to spawn any type of killer app capable of lifting it out of obscurity.
HTML5 fails with a shortsighted set of 2D transformations. The seemingly simple addition of a third component / row / column for the new HTML5 canvas element being left to Apple where webkit chooses to adopt it's own standard.
This hardware frugal author is unwilling to investigate further. Prototyping in Flash10 has begun...
Khronos enabling hardware-accelerated 3D graphics in Web pages without the need for browser plug-ins.
Haxe provides a fundamental abstraction to all the technologies and more for the site coder.