HTML and CSS are at the heart of nearly all web interfaces. (Yes, you can build almost everything in Flash, for example, but it’s rarely done.) Making effective use of HTML and CSS is challenging for a variety of reasons. The standards, even if perfectly implemented, have a variety of shortcomings. Complicating the picture are the differences in how each browser implements, and deviates from, the standards. Fortunately, there’s a lot of published web design lore that leads the way.
A list of essential reading for people interested learning CSS. Most of the articles are from A List Apart, but the volume of articles at A List Apart can can be overwhelming to people just to getting started. Consider this a pocket guide to the best of A List Apart with a few additional links thrown in to spice it up a bit.