Ask HN: Best Coding Kata website?

I'm a big fan of Codewars (http://www.codewars.com/). I'm not sure if I'd consider it Kata though.

Signup seems to not be working for me. I keep seeing a bar on the top that says "undefined method `[]' for nil:NilClass"

Pretty cool. Is this just javascript based?

Yeah, with Rails on the backend.

Really good. Is there a collection of basic "industry" katas to start with that you are aware?