I don't feel likely to get much out of typing (+ 2 2) and getting 4 in a web REPL. What I want is a 1-page writeup with basics about the implementation: Lisp dialect, implementation strategy, special features and limitations, how the concurrency works, resource requirements, source code SLOC, stuff like that. It's ok for the writeup to assume I'm familiar with Lisp. I looked at the web site for a few seconds and the home page was almost entirely uninformative. I didn't try reading the documentation but might look at it later.

The home page of ulisp.com isn't exactly what I want either, but it's a lot closer.