1 - You could use C# (provided that a compiler exists), but this only gets you the language, not the APIs and libraries commonly used by it. In that sense, it's not much different from using Emscripten (a compiler that goes from other LLVM languages to JavaScript) today. For example, XAML won't work unless someone writes a library to translate it into the browser's usual structures. 2 - It's still JavaScript underneath (more accurately, it's asm.js underneath).