CSS coding, like most UI coding, is at least part voodoo, what with browser incompatibilities and the fact that a good number of developers have avoided it until every became Ajax-ified.
If you’re looking to learn CSS or refine your CSS chops, make sure you check out Smashing Magazine’s 70 Expert Ideas for CSS Coding. It provides tips of all kinds, from the general (such as workflow) to the esoteric (such as dealing with Internet Explorer).