It is kind of soul crushing to work on a problem only to be denied execution. Thankfully, I ran an easy JS challenge that was a one-liner. Had I done a a harder challenge that took 10 or 20 minutes, I would have been rather upset.

I understand why you put the registration further into the process. People have already invested time, so they might as well register so that the time isn't wasted. This is markedly different from how the rest are. They either put registration up front (HackerRank) or you're required to validate your own code (cryptopals). Aside from intentionally infecting my computer with malware or something, I couldn't think of a more negative user experience from a code challenge site.

These types of sites are far from unique. They are basically a dime a dozen. But now my experience is, "Don't make the same mistake I did. It gives you an input field to submit a challenge, but it will not validate your answer until you register. Either register up front, or don't bother."

Edit: This post was intended to be constructive, but I can see how it can be interpreted otherwise. See here[0] for more info. Apologies!

[0]: https://news.ycombinator.com/item?id=14349780