As you validate your stylesheet, you may often get the congratulations that it validated, but you end up with tons of warnings...
Since W3 modified the css validation service, a new fixture was added some time ago (don't know exactly, but it's been here for quite a while now..).
I experienced it when I was validating a site, my stylesheet always validates without a problem, but I got a lot of warnings, saying I didn't specify the background colour for a number of elements..
First of all the reason why this warning shows up is that if you for example are loading an image as a background colour, and this doesnt load, the user might see white text on a white background, as white is the default color for a background colour.
Another thing that might happen is when a user uses a personalized stylesheet on his pc (for example if he has trouble staring at a white screen for too long). If he has the setting that the background is black, and you specified your textcolour to be black (color:#000;), then he ends up with a black screen..
On a side note: I saw this on a pc of one of my clients, he changed the colour settings of his pc, and when I purchased SwishMax on his pc, the form at swishzone was unvisible, so I had to select the text in order to see it.. Not that it mattered to me, but people who have less patience might become annoyed and click-away. Thats 80 euros down the drain per copy
So what to do when you really cant specify a background colour, because you need the element to be transparent, when for example the background image of the surrounding element needs to be visible?
CODE
background-color : inherit;

That's all! Just place this code for the element that generates the warning, and the stylesheet will validate without a single warning!
Now don't add this line to every single line, it's better to first look at the warnings (it specifies the line and the element on your stylesheet), and fix it according to this list. If you for example add this line to an a:hover element, you get new warnings saying it contains a block element etc. and thats not what we want as we are getting rid of the warnings!
Lot of text for a single line of code, but just to make a point and explain the reasoning behind it...
Happy styling!