I started this project to learn javascript and HTML from scratch, since I had only ever had exposure to framework-based web dev. The user code is sandboxed, sanitized, and evaluated in real-time. Please let me know if you have any tricks for breaking out of the sandbox and hacking the game or modifying the DOM, doing AJAX requests etc.
This is all done with vanilla js + a small stateless component library for the inspector panels. I had a lot of fun with it. I am working on multiplayer tournament servers with ELO ranking, but I don't have the time to finish a full website with login and security etc.