CSS Level 1
The original CSS Level 1 recommendation was released by the W3C in December 1996. There have been many revisions since its original release. You can find the original recommendation and the latest revision at w3.org
CSS Level 2
The CSS Level 2 recommendation was originally released by the W3C in May 1998. There have been many revisions since its original release. CSS Level 2 has had two revisions, CSS 2.1 and CSS 2.2.
CSS Level 3 and Beyond
The CSS Level 3 recommendation was released in May 2001. Since its release, there have been several changes, including the method for making changes. For the latest updates, visit the CSS Current Work webpage.