There's a performance problem. The open Web only works because we have enough compute power to tie up multiprocessor computers with a few gigaflops per CPU and gigabytes of RAM to display 2D text with a few pictures on the screen. A good 3D game world pushes the limits of what current hardware can do. The level of inefficiency associated with Javascript/HTML/CSS won't work for an interesting 3D world. It was tried. See X3D.