CSS4

CSS4 is not a single formal specification. In the history of CSS, the language has evolved through a series of modules rather than a monolithic release. After CSS2.1, the CSS Working Group adopted a modular approach to CSS, with individual modules developed and published as separate documents. The term CSS4 has been used informally to refer to the collection of upcoming features and modules that would extend CSS beyond CSS3.

Because CSS4 is not a unified spec, there is no single set of features labeled CSS4. Instead, features are published as modular Level 3/4 drafts, such as Color Module Level 4, Selectors Level 4, Functions and Units Level 4, and layout-related modules. Some proposed features gained broad browser support while others remained in draft form. The modular approach allows independent timelines for specification and implementation.

Impact and status: Web developers typically rely on features that exist in individual modules rather than CSS4 as a whole. As features graduate from Working Drafts to Candidate or Recommended status, browser support becomes uneven, and progressive enhancement is common. The term "CSS4" is often used in historical or speculative contexts and is less used in official documentation today.

See also: The CSS Working Group and CSS Modules.