Thanks, we are pretty excited about it! :-) We use WebGL for rendering on the client side. We have a custom 3D engine written just for Tinkercad. Our main UI guy is an ex game graphics/AI programmer http://digestingduck.blogspot.com/. He's done stuff like the Zen Bound iPhone/iPad game and AI for Crysis.

If you are interested in seeing the rendering technology our viewer works for everyone, take a look at this bunny model: https://tinkercad.com/p/acc048395e191555