As you design your Theme or style, you will be using HTML and CSS references. Here are a few resources you will become very familiar with over time. We recommend that you save or bookmark this page as a reference while you're working on your WordPress Themes.
If you want to learn more about CSS, especially cutting edge CSS and web page designs, visit one of these forums and mailing lists to share your thoughts and learn from others.
The following are some of the best books to help you learn more about CSS and web page design.