So it basically means that C++ got another reasonable compilation target. Not much to do with applying JavaScript to large-scale games. Also it is not HTML 5, it is HTML FF23.
Factually, all code emitted from emscripten is JavaScript. It may look funny with all the bitwise operators that are barely ever used by web developers, but it's just JavaScript. It doesn't run correctly in Chrome right now because they have issues with their JavaScript environment not supporting large numbers of variables for a given context. It doesn't run in IE because IE doesn't support webGL. Both of these will be fixed in the future.