The result of three and a half days of porting, RetroArch is now on your web browser! "How useless!" you say? Well probably, but it was a fun thing to do.
Most of the general features of RetroArch are here with some exceptions. Sound is not in yet; waiting on this pull request to get accepted before trying to get it working, and there are some reports that Firefox has issues with gapless audio. V-sync is based off of your browser, so it's not perfect. Also, file saving is not in yet since I don't want to write a bunch of Javascript right now.
Firefox 22 or later gets the best results with asm.js, especially if you enable native GL (in about:config, set "webgl.prefer-native-gl" to true). Google Chrome is not as fast, and I struggle to get fullspeed on an i7 4770. I've heard that nightly Chrome has made some big improvements in compiled Javascript execution, so that might be worth trying. Other browsers may or may not work, but don't bet on them.
Coming soon: Mupen64 port with Javascript Dynrec? (don't count on it)