Instead of spending ages building geometry for your 3D worlds, why not just define a mathematical 'field' function that uses fractals and folded space and pseudo-random noise and all kinds of other tricks? It certainly saves on the disk space.
Think of it as a kind of disguised Mandelbrot set. You can generate infinite variation from a carefully iterated function, and often you can zoom all the way down and variation just keeps appearing.
The hard part is combining these 'shader' backgrounds with actual geometry. Until recently, this wasn't possible to do in the browser, but a new WebGL extension (EXT_frag_depth) allows you to control the 'fragment depth' of individual pixels, (ie: skyboxes can now push parts of themselves into the foreground.)
Fortunately, it's a trick I've managed to implement in Astromech.
So now, I can do this kind of thing: take a classic 3D model (saved to COLLADA format from blender) and compose it with a procedurally generated 'skybox' that I call a 'skyshader'.
I hope to do a video shortly, showing off these fully animated 3D environments.