Relative lengths/widths are better than absolute; if you absolutely have to use absolute, make sure you check in all the different site skins available and at both 6pt font and at 60pt font to make sure nothing breaks. (We have people who use both extremes.)

Tables-for-layout: don't. Tables-for-tabular-data: okay, but be sure to use <th> and labels.

(Most of these are less CSS guidelines and more accessibility guidelines, actually. We don't have an official style guide for it, but we should.)