Here is a very early demo of a 3D game engine, written in C++ and using OpenGL, compiled to JavaScript and WebGL using Emscripten. The game engine is Sauerbraten (aka Cube 2), one of the best open source game engines out there, and we nicknamed the port BananaBread. After loading the demo link, press the "fullscreen" button, then click "GO!" to start the game. Move with WASD, jump with space, look