This is fun! Just yesterday I was thinking of creating something similar, but your version is great. I especially like the "keystrokes used" in the leader board to learn from and the diff view. Looking forward to some hard challenges.

My suggestions:

- It should show which challenges I already solved in the list of challenges, but it does not. I did not log in, maybe this is different then.

- Navigation between challenges could be simpler. Something like a button "go to next unsolved challenge".

- You could display the leader board when a challenge was solved. You could even include the keystrokes used in the leader board, without the need to click on each position individually. This makes it easier to learn from better players.

- I find the "Keystrokes used" a bit hard to parse because of the comma between each key. A monospace font would be nice as well.

- I suspect "Rank by Time" will become meaningless for most players when this gains popularity and and people submit unbeatable scores by cheating. Maybe private leader boards I can share with my friends would be a nice addition.

- For a vim-based game, I need to use the mouse too much for navigation.

I will definitely come back to this for more challenges.