The first implementation of CSS was drawn up in 1996, and released in 1999; it has been supported by all browser releases since 2001. The standard for this version (CSS1) was revised in 2008. Then, beginning in 1998, developers began drawing up the second specification (CSS2); its standard was completed in 2007 and revised in 2009.
Development for the CSS3 specification commenced in 2001, with some features being proposed in 2009, and the latest recommendations being included as recently as 2016.
A CSS4 is already being proposed by the CSS working group, although this is not a major leap forward. Rather, it’s simply a development of one part of CSS—the selectors—and its contents are beyond the scope of this book, given that the first draft discussion documents are only just being published as I write this edition.
Thankfully, though, the CSS working group publishes regular snapshots of the CSS modules that it considers stable. So far, four such best-current-practices documents have been published as Notes, with the most recent being in 2017. This is the best place to gauge the current state of play in the world of CSS.