This is really cool, nice work!

A couple of minor criticisms (as an intermediate Vim user): - I felt like it was too easy to just stare at the key instruction on the top left and just ignore how you're navigating the block of text itself. As you are letting people know what commands are being introduced in the next lesson, maybe they should only be shown if the student wants a hint? - It seems like the only way to go to the next lesson on completion (without using the mouse) is to tab once then hit enter. Maybe that could be a keyboard command as well? `n` maybe?