Performance for simple things is adequate, but relying on 2-point rectangles (origin(x,y), dimensions(width,height)) for geometry is extremely cumbersome. Also everything is hack piled on top of hack (for example there is no concept of a camera, the whole world moves around you instead to simulate the camera effect!).

So... webgl is still the way! This is meant to be an experiment to show what css/html is capable of!

It won't be faster than WebGL in most cases. Browser CSS rendering stacks have gotten pretty fast but are not really optimized for this. In practice WebGL gives you more chances for optimization, because CSS rendering has to handle lots of edge cases that games don't. For example, a CSS renderer must split self-intersecting partially-transparent 3D polygons on CPU to render correctly, whereas a game engine just tells the artists "don't do that".

I think it's pretty hard to get more performant than webgl, especially as the number of vertices increases/geometry complexity increases.