[cyral]

The text-overflow one really gets me every time... I would also add stacking contexts and margin collapse to this list. The rules for when they apply and when they don't are extremely convoluted and full of exceptions.

One note, there are "rems" which are ems relative to the root. I love ems and use them for almost every component because of the size inheritance actually. rems are used when I need to make sure something is always the same size regardless of the parent, but that doesn't happen often.