Why There Will Be No CSS4: The Move to Modular CSS

CSS3 will not evolve into a CSS4 version; instead, the CSS specification has been broken into many independent modules that are upgraded separately, so there will be no monolithic CSS4.

Since CSS3, the specification has been split into modules, each advancing on its own or being created as a new module, which means future changes will be expressed as module-level updates rather than a new major version.

Historically, CSS did not have version numbers but "levels" (e.g., CSS Level 1, Level 2, Level 3). CSS1 is obsolete, CSS2 is effectively obsolete, and CSS2.1 is a revision of CSS2 that removed immature content and sent it back to the Candidate Recommendation stage, later re‑emerging as part of CSS Level 3 modules.

Because earlier CSS specifications were monolithic documents (CSS2.1 PDF is 430 pages), maintaining and upgrading them became cumbersome. The CSS Working Group therefore adopted a modular approach after CSS2.1.

Working Draft (WD)

Last Call Working Draft (LC/LCWD)

Candidate Recommendation (CR)

Proposed Recommendation (PR)

Recommendation (REC)

Modules before modularization (originally from CSS Level 2) include Selectors, Color, Values and Units, Backgrounds and Borders, etc., and they were renamed to start at Level 3 (e.g., Selectors Level 3). New modules such as Multi‑column Layout, Transitions, Flexible Box, and Transforms start at Level 1.

Modules that were removed from CSS2.1 are treated as having reverted to the CR stage and may reappear as part of CSS Level 3 modules.

Even though the overall CSS language will not have a Level 4, individual modules can reach Level 4 or higher; for example, the CSS Color Module is progressing to Level 4.

There is no CSS Level 4. Independent modules can reach level 4 or beyond, but CSS the language no longer has levels. ("CSS Level 3" as a term is used only to differentiate it from the previous monolithic versions.)

Useful reference links:

CSS snapshot listing current stable modules: https://www.w3.org/TR/css-2017/

Complete list of CSS modules: https://www.w3.org/Style/CSS/current-work

Introductory guide by W3C expert fantasai: https://www.w3.org/Style/CSS/read

Feel free to leave comments if anything is unclear.