Google has been working on a GWT (Google Web Technologies) port of Quake II, meaning that they can natively run the game in a webbrowser that supports HTML5. This means that it is actually the browser rendering the game and not some sort of executable OS-native plugin like Quake Live. They use WebGL for hardware acceleration.
Below is a video running this HTML5 Q2 engine. They claim framerates up to 60fps but the video doesn't quite show it