Mind blown. It's time to learn how to use WebGL.

Except it was written in C++ then converted to Javascript/WebGL.

The mere fact that even that is possible is pretty mind-blowing.

Agreed.

The math for calculating the mouse pointer position seems off, even in full screen mode.

I think the engine multiplies mouse movement by some factor, so the OS cursor diverges from the game cursor. But in fullscreen+mouselock mode it should work fine, not sure why not in that link, but here is a link to another wip port of the same game engine to JS/WebGL,

http://syntensity.com/static/bb/client.html

In fullscreen mode the mouse cursor works ok (at least in browsers that support mouselock).