The actual REPL is the least impressive thing about replit!
If you use an HTTP server it will automatically route whatever port you opened to 443 on its own replit.co subdomain. If you use a program with an X11 context, replit will automatically detect that and render it in its own in-page visual context; plus they allow VNC connections to that environment via NoVNC. They have an HTTP API that allows repl-instance-specific persistent storage.
I'm not a paying customer or an employee or anything but I gotta say they're pretty innovative. (I will agree with others that the documentation IS definitely their weakest point, but that in part because they have so many features.)