I've been busy finishing off and recording my talk for the upcoming An Event Apart Spring Summit. The aim with the talk is to explain some of the core concepts of CSS, to help make everything else seem more understandable.
I was keen to see if there were things I had missed, and so posted this question on Twitter. It took on a bit of a life of its own, with fantasai giving some very detailed answers to many of them. It was also very useful for the work I do on MDN, on the CSS docs. Knowing where people are confused helps me to know what should be better explained in the docs.