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

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".