CSS, with the layout showing

Forty steps through flexbox, twenty-two builds that fail unless several concepts are right at once, and the famous wtfs underneath — margin collapsing, stacking contexts, and the invisible wall inside every flex item. This is the one subject where the output IS the picture, so every problem is rendered rather than described.

Problems

5 problems in The famous WTFs

All topics Advanced Alignment Distributing space Gap Main axis vs cross axis Making a flex container Order Sizing The famous WTFs The min-width:auto floor Wrapping and the align-content trap

Easy Medium Hard|Teaching step Predict the render Build it Explain the wtf

#	title	topic	type	difficulty
1	Margin collapsing, and the margin that escapes	The famous WTFs	Explain the wtf	Hard
2	Block formatting contexts	The famous WTFs	Explain the wtf	Hard
3	z-index: 9999 doesn't work	The famous WTFs	Explain the wtf	Hard
4	Percentage padding resolves against width — even vertically	The famous WTFs	Explain the wtf	Hard
5	display: contents removes the box	The famous WTFs	Explain the wtf	Hard

44 of 87 catalogue topics are authored, with 37 drills rehearsing them. Every one of them is judged: each carries a starter that has been shown to fail, at least two different correct answers that have been shown to pass, and a set of near-misses and cheats that have been shown to be caught. Nothing reaches this list until all of that holds.