Flexbox Froggy: A game for learning CSS flexbox
The possible values for justify-content are only displayed on Levels 1 and 2. By Level 3 you're supposed to have all 5 values memorized even though you've only used 2 of them? I had to keep flipping back to Level 2 just to get a list of the possible values. For a game built for someone who has never used flex before, I think you're assuming too much familiarity with it.