If you use things in your CSS that begin with -moz or -webkit, etc. you will probably get some errors on these during validation. If you are validating manually on W3C's Jigsaw service, simply hit "More Options" and change "Vendor Extensions" to "Warnings." Presto, gone. Additionally, CSS3 is listed in the same place.
The funny thing is, no matter how you set your options on that page, the CSS button it gives you is
NOT going to follow that. It will follow the defaults. However, you can manually change the link to your liking. Here's an example:
http://jigsaw.w3.org/css-validator/c...xtwarning=true
Notice the GET variables profile=css3 and vextwarning=true. These will make sure that W3C validates your stylesheet correctly.
Additionally, if you're using HTML5, you can find a nice valid icon
here, which also provides a similar CSS one.
Hope that helps!