“Because of the generally crappy CSS standards support in modern browsers, a number of techniques have been developed to hide CSS stylesheets, rules, and declarations from browsers that won't understand or will mis-interpret them, or to set different values to the same CSS property in different browsers in order to acheive the same appearance on all (or most) browsers. On these pages, I've collected a number of these techniques (usually called filters or hacks) and documented what browsers they work on. The W3C validator may reject code that appears to follow the letter of the emerging CSS3 standards. In such cases, I've assumed that the filter is valid CSS. In the summary tables, the background color of the CSS Version column indicates whether each filter passes the W3 validator or not (green = validates; red = doesn't) so that you can stick to filters that have been given this official seal of approval (I've removed all the obviously non-valid hacks to their own section).“
This is a nice link to have in your web developers toolbox.