The result of three and a half days of porting (plus a couple weeks of adding features), RetroArch is now on your web browser! "How useless!" you say? Well probably, but it was a fun thing to do.
Firefox 22 or later gets the best results with asm.js, especially Nightly versions. Both Chrome and Firefox 25 and later will have audio support, but earlier versions of Firefox will not. You can also get better performance on Windows if you enable native GL (in about:config, set "webgl.prefer-native-gl" to true) but this might not work on all drivers *coughamdcough*. Google Chrome is not as fast, but you may get better audio on it. Even though Internet Explorer 11 has WebGL support, it is not complete and RetroArch will not work with it. Other browsers may or may not work, but don't bet on them.