Level One warnings for CSS - help

Hi folks

Can someone provide a good link that explains Level One warnings for CSS
files.

I've started to get my css validated at the w3c site and I've been
getting these level one warnings - specifically setting the border
colour to be the same as a <div>'s background colour is a no-no (I
changed it to achieve validation but I don't follow the logic behind it)

I'm a bit mystified as to what these mean and would like to read up on
it - I did a search around the w3c site - but I find that the sheer
volme of results was too overwhelming.

Any links you know of?

Thanks

Alex