Automatic Method:
You should now simply use: https://jigsaw.w3.org/css-validator/check/referer?profile=css3.
Manual Method:
Typically, if we want to link dynamically to the W3C Valid CSS page, we use this link:
https://jigsaw.w3.org/css-validator/check/referer
That’s perfect if you’re using all CSS2.1, as that’s the default checker, but if you implement even one CSS3 style, you’ll fail validation. So, we can no longer use the referrer method until CSS3 becomes highly supported to the mainstream.
As a compromise, instead of linking our pages to the referrer link, we’ll have to link our stylesheet to the CSS3 Validator like so:
<a href="http://jigsaw.w3.org/css-validator/validator?uri=http://website.com/css/style.css&profile=css3">Valid CSS</a>(remember to use & in place of & so you don’t break HTML validation)