W3C’s CSS validator will suggest a button that validates your CSS as CSS 2.1. You will need to change the deafault button if using any CSS3 code to prevent any errors showing.

So if your ahead of the curve by using CSS3 you can still sport the valid code banner!

Default CSS validator button

<p>
    <a
    href="http://jigsaw.w3.org/css-validator/check/referer">
        <img style="border:0;width:88px;height:31px"
        src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!" />
    </a>
</p>

CSS3 validator button

<p>
    <a
    href="http://jigsaw.w3.org/css-validator/check/referer?profile=css3">
        <img style="border:0;width:88px;height:31px"
        src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!" />
    </a>
</p>

It’s a simple change, but the my innate OCD just loves the green “Congratulations! No Error Found.” message!