In a rush, what would help me refresh flexbox knowledge is being able to drag and drop the frog, snapped to a 3x3 grid, and get the flexbox CSS code as output.
Still a very thoughtful tutorial.
(EDIT: The reason I said the above is I've personally given up on keeping up with new CSS stuff. Having written CSS off and on since IE7 days, basically I have no energy to keep css tricks in memory, having lived through hand coding vendor prefix flags and whatever the new Chrome Canary is thrusting down the pipe)