Unlike CSS 2, which is a large single specification defining various features, CSS 3 is divided into several separate documents called “modules”. Each module adds new capabilities or extends features defined in CSS 2, over preserving backward compatibility. Work on CSS level 3 started around the time of publication of the original CSS 2 recommendation. The earliest CSS 3 drafts were published in June 1999.
Due to the modularization, different modules have different stability and statuses. As of November 2011, there are over fifty CSS modules published from the CSS Working Group. Three of them―Selectors Level 3, Namespaces and Color― became W3C Recommendations in 2011.
Some modules (including Backgrounds and Borders, Media Queries, and Multi-column Layout among others) have Candidate Recommendation (CR) status and are considered moderately stable. At CR stage, implementations are advised to drop vendor prefixes.