Prime numbers, semi-transparent PNGs, and organic randomness in tiled backgrounds: The Cicada Principle and Why It Matters to Web Designers. Wish I had an excuse to use this.
In which I realize I stopped learning new CSS tricks a few years ago: CSS: Innovative Techniques and Practical Solutions (via)
Good introduction by Dan Cederholm.
A simple Web application helps you optimize your stylesheet — or revert to a readable format as necessary.
Easy little CSS library to use when you want to include diagrams of keyboard commands on an HTML page. Like, “Then, press Cmd+Shift+S to Save As a .png”
Border tricks make triangles show up in interesting (and useful) places.