Looks like this uses the Truck modeling kernel: https://github.com/ricosjp/truck

Interestingly, ttouch created browser-based CAD tool CADmium a few years ago based on Truck. Then, a few months ago, they created another one (CADara) based on OpenCASCADE: https://news.ycombinator.com/item?id=48070083.

Truck seems cool, but creating a b-rep kernel is a phenomenal amount of work. Given that, I'm curious how quickly SolidCraft will mature.