I think CSS itself isn't so complicated, but the whole different support in different browsers thing certainly does make a mess of it!

One of the most helpful things I ever did with respect to CSS was read the apress book on html/css patterns (just ignore the parts that use javascript).

http://books.google.com/books?id=xq9IOomk1tUC

(I never realized that standard CSS actually operates with 6 box models!)