Nope, threejs is used for a very specific thing at the end of the second level. There is actually nothing 3D-related webgl in it (except for a surprise at the end)
Couldn't figure out, how you did that in CSS. Now I know that a little bit of cheating was involved ;)
It's billboarding! (and the css lightsources are duplicated in the webgl world, so that the worm is properly shaded)
I don't think it's there any more, but many versions ago three.js included a CSS renderer. It was very useful for making things line up on 3d elements of webpages.