Mozilla engineers for a while have been toying with porting games/engines to the web browser using WebGL and Emscripten. Last month Mozilla announced they brought the Unigine Engine to the web while their most recent porting effort revolved around Unreal Engine 3.
This port should work in any modern HTML5 web-browser with WebGL acceleration. Mozilla is also said to be working with other game publishers like Disney and EA.
More details on the Unreal Engine 3 work for the web can be found via this Mozilla blog post and the video embedded below.
The port reportedly took only four days due to using Emscripten and asm.js.