Here is a collection of clever things I've learned about CSS:
Introduction to CSS - Including how styles sheets work, and how to change the appearance of links.
Printing and CSS - How to make the printed version of a webpage look different to the on-screen version. Also how to put table headers at the top of each page when printing a long table, and how to force a page-break.
Changing font sizes - Font sizes, small caps, and condensed fonts.