Seems like VRML would've worked, huh. It looks like they do have an export to .stl, and in fact it looks like their web viewer actually uses .stl with three.js

Seems like VRML would've worked, huh. It looks like they do have an export to .stl, and in fact it looks like their web viewer actually uses .stl with three.js to represent the model. So there's that. Almost the rest of the comment is just random observations that have little to do with your observation. I think they did it this way because they are going for a bit of a Markdown approach. HEF looks somewhat more human readable than VRML once you learn the syntax and they have a basic tutorial on their home page. I fed one of their example pieces into their online viewer and it loaded instantly. I was hoping for a parts list from it but I don't see one. Aha, that's because it's embedded in the .hec itself. Not knowing VRML I'm thinking I could build something with .hec, and I can't build my way out of a paper bag.