Cascading Style Sheet specifications, unlike HTML specifications, are backward compatible. That is, if a file conforms to CSS1 it also conforms to CSS2 (but not vice versa). So conformance to any CSSx is sufficient to comply with web design standards. I have chosen to stick with CSS1 usage because, here in mid-2002, CSS2 has not yet achieved widespread usage. Indeed, though CSS3 is under development, even CSS1 is not even implemented quite right in Internet Explorer 5.5. See the screenshot of an IE 5.5 browser displaying a W3C test page comparing CSS1 and CSS2; you can check your own browser here. See the CSS reference for more.
Cascading style sheets are a tremendous boon to web standardization and development, but it is a young technology. The various browsers are CSS-compliant to varying degrees. For testing purposes, you might want to look at the Opera browser, which is widely regarded as adhering to standards much more closely than the big guns.
Just as with HTML, you should validate your style sheets for conformance to web standards. The W3C, as it does for HTML, has a web-based CSS validator which you can use to check your style sheets. The Web Design group also has a web-based CSSCheck utility. Some webpage editors provide CSS validation. Bradsoft.com's Top Style tool even provides validation against different versions of several browsers. Once validated, you may add the W3C seal of approval to your page, an image indicating that your webpage conforms to web standards. This image is available through the W3C validator once your page successfully validates.