The core of C3 is a custom-built framework we designed and developed ourselves, because we didn't think any other Javascript libraries out there were suitable. It's a pretty interesting part of the development, I'm hoping on doing some tech blogs about it in future.
That's awesome! Based on your earlier blog posts, I had assumed that might be the case.
I wrote my own non-standard framework as well (based on async single file components without a build process).
I think I have a vague sense of the sheer amount of work and technical prowess required to build a browser-based Construct 3. I am definitely looking forward to any posts you make regarding the system you created. I'm also not aware (off-hand) of any browser-based applications pushing the envelope quite so far!