Just one observation from the results to the first challenge: submission without output for the example input (either given in the box or as link) are not so nice because one would need to run the program oneself. Maybe require that if the output can be variable (it was in challenge 1).

Also there should be some accompanying text. What is the name of the language, what technique was used? After all readers may want to learn something. Maybe also require this.

And what about a period reserved for clarifications (like asking for the character set to encode in challenge 2) with comments below the challenge. There should be a time reserved for this before the clock officially ticks for solving the challenge.