That depends on the code. Neverball has acquired a high level of portability thanks in large part to SDL2 and OpenGL (ES). So compiling to Emscripten was mostly about rearranging the pieces that were already there. My original patch was surprisingly small. (https://github.com/Neverball/neverball/commit/517c93b7bcf63f...) I would say I spent more time on integration with browser APIs, so it would behave and feel more like a web app.
