Update: please use this link instead, it's a newer version of the demo, and HTTP requests are served more quickly: http://n3emscripten.appspot.com/
Chrome and Firefox should simply work. In Safari on OSX you need to enable WebGL support first. Internet Explorer won't work since MS thinks WebGL is evil.
Once I have the same functionality in the emscripten demo as in the NaCl demo, I'll write a little "postmortem" on the emscripten port.