Conceptually: I like the idea of giving some personality to the "type here and see something happen" code-learning toolkit, at least for the HTML/CSS course. Not sure how well this specific concept will go down with the entire range of K-12--I see this working well for grades 4-8 boys. (I teach programming to grades 8-11.)
Some specific issues:
- I'm a little worried about how the site throws a lot of (presumably unlicensed, at the moment) brands at the user. The Avengers logo, Samsung-branded phone in the HTML/CSS course, the DC Flash icon...
- Some essential lesson navigation elements ("results" in the JS course, "next" in both courses) are quite far down the page, which isn't great for users with small screens. A browser maximised for a 900px-height screen seems to work ok, but any smaller and the learner will have trouble finding the "next lesson" button, which can't be much fun.
- HTML/CSS course: Getting the learner to hit "check" immediately sometimes hides the result that would have shown up in the "browser". This could be confusing.
All the best!