The dashboard UI is pretty confusing, I switched to Chrome because I thought it was broken in Firefox, and in Chrome the "preview" (now I know it is a preview) has shadow on the bottom. It wasn't at all clear to me that clicking 'Train' would start the exercise with that preview. Otherwise, massive kudos, I hope you upgrade to CodeMirror 4 with multiple cursors and improve the CoffeeScript highlighting (I really feel Ace is more ready for what you're doing). All the best!