CSS dos and don'ts

When you're looking at lists of dos and don'ts for CSS you usually think of just the CSS selectors, using external style sheets, and other aspects of the CSS. But as CSS Globe points out, structuring your HTML markup is just as important as the CSS. These CSS dos and don'ts for markup are great suggestions for how to improve your code. One I think more people should keep in mind is "don't think that if something is valid it is automatically web standards compliant."