Peter Paul Koch, in Digital Web Magazine: Keep CSS Simple:

CSS hacks are actively dangerous for your Web site. A cursory glance at the underlying “theory” (if one can call it that) will rapidly convince you that the average CSS hacker doesn’t have the faintest idea what he’s doing.

A certain browser has a certain CSS bug. Good to know. This same browser has another bug, usually in its parsing of CSS selectors or comments. This, too, is important information. However, a CSS hacker proceeds to use the second bug to “solve” the first one.

In an ideal world the next release of the browser would solve both bugs. In an uncaring world the next release of the browser would solve neither. In the uncertain world we live in the next release could solve one bug but not the other!

PPK has also just started QuirksMode.org.