If you are using C, you can afford to compute everything at low level as Notch is doing in this example. And you can afford writing pixel by pixel into a frame

If you are using C, you can afford to compute everything at low level as Notch is doing in this example. And you can afford writing pixel by pixel into a frame buffer that is later rendered. However if you use this approach in Javascript you get poor performances unfortunately. And indeed this demo requires fast computers to do what otherwise could be done in C with direct access to the video frame buffer with maybe 50 times slower computers. Even C coders almost no longer try to do this low level things and let the graphic chip do a lot of the work when the goal is to produce a game. So in Javascript instead you want to usually call a library that implements a 3D engine in a lower level language like C and with more direct hardware access. But in this case, it makes sense to do all this in a low level way and in Javascript: it is just a demo that is full of interesting code and with an appealing visual output.