W3 Schools has the best online resource for coding. Not just for CSS!
Make sure you keep your knowledge up with new versions of CSS over the years.
If your CSS isn't fully compliant, its inevitable that some of your users (if not all) will not experience your website the way you want them to. TESTING PEOPLE, TESTING!
This wont teach you everything you need to know, but it will certainly help reinforce the 'basics' of CSS.
This is a blog that will spit out some more advanced techniques now and then.