Rachel Andrew explaining CSS Levels:
If you read something about CSS3 Selectors, then what is actually being described is something that is part of the CSS Selectors Level 3 specification. In fact CSS Selectors is one of the specifications that is marked as completed and a Recommendation. The CSS Working Group is now working on Selectors Level 4 with new proposed features plus the selectors that were part of Level 3 (and CSS 1 and 2). It’s not CSS4, but Level 4 of a single specification. One small part of CSS.
In the comments of the article Šime Vidas posted a link where "you can check which modules have multiple levels and which one is “Current Work”." Very useful.
