One common problem in CSS that causes warnings in validators is to have a color property with no matching background-color property.
Here's a good article about this.
At first blush, this might seem to be a pretty superfluous warning message, but if the user has a user-defined CSS that is overriding your CSS, for example, a visually impaired user, specifying one or the other and not both can cause some big headaches.
Imagine, for example, that you set "background-color" to black, and don't have a "color" set. If the user overrides your "color" object with the color black, they won't be able to see the text.
So, I went through and made pairs of all my background-color and color specifications, and cleared up a whole bunch of warning messages.