The CSS 2.1 Properties Reference from Cultured Code provides a compact package of CSS specs and their meanings.
An interesting little tidbit I picked up today whilst banging my head against my desk: when Firefox renders a page in Quirks mode, unordered lists that specify ‘disc’ as the bullet of choise are instead rendered with diamonds.
The post Write Stylesheets Simultaneously has a great tip for easing the initial development of a new site. Instead of manually reloading your browser each time you want to view your changes to your markup or CSS, throw a simple meta refresh into the HEAD so the page will reload automatically every N seconds. A second tip is provided a bit farther down the page – so check it out.
Nifty Corners Cube is an image-free way of building rounded corners!
Eric Meyer sheds some light on the interesting application of line-height within the cascade, specifically he recommends implementing Unitless line-heights (‘1’ instead of ‘1em’ or ‘100%’ for example) in order to avoid the wrong values flowing down the cascade to descendant elements. I highly recommend you check it out should you be a Web dev type of person.