It's actually really easy to build C interfaces to any Javascript API or lib. Since compiled code is just JS you can just directly call from C code into JS code and vice-versa. Most of emscripten's CRT wrapper is actually implemented in hand-written JS, as well as the OpenGL-to-WebGL wrapper. For an app which mainly needs to do DOM manipulation it is probably better to write it mostly in traditional JS and only implement the performance sensitive stuff in a C library compiled to JS. Best of both worlds...
