So you just take previously written C/OPENGL games/prototypes and you compile them for the BROWSER!!! It's madness!
I don't have the nerve for web programming, for tons of DOM elements and css, for different js vm implementations, for debugging hell and everything else modern web "gave" me as a developer. And now I don't have to deal with all these things, and still I can deploy for the browser!!! ( okay, I have to deal with them a little because it still is web development, but hey, I only need hours now to fix something strange, not days!!! )
IMHO Emscripten is the best technology of the 2010's so far.