While referring to all new CSS as CSS3 worked for a short time, it doesn’t reflect the reality of where CSS is today. 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.