Additionally, there are often many ways to accomplish the same result but fully understanding the implications of certain approaches over others is also a headache at times.
Speaking personally, something touched on in this article, is that many things don't really make sense to me or at least I don't find them intuitive - and I've been working with CSS daily for 14 years. It does often feel like a scatterbrained, horse designed by committee type of thing.
It's like CSS only makes sense when you learn to see the problem through the eyes of whomever specced out that particular feature, which can also be said for trying to read CSS written by another developer.