As CSS has grown in complexity, and new features are added, it doesn’t make sense to draw a line at which all work is stopped on all parts of CSS in order to declare that CSS version finished.
It’s a hard topic to understand what differs between different CSS versions. As Rachel explains perfectly in this article, CSS has not been a version of itself since 2.1. It’s actually smaller modules that makes up CSS, and their versioning is completely independent of each other. She continues:
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.
And then with:
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.