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This slide deck gives a quick overview of the W3C specification maturity process and the various levels of CSS.
The World Wide Web Consortium or the W3C is an international community that develops open standards for the Web.
The W3C produces specifications on a wide range of web-related topics (including HTML and CSS).
W3C technical specifications have four levels of maturity:
A document that W3C has published for review by the community, including W3C Members, the public, and other technical organizations.
A document that satisfies the Working Group's technical requirements, and has already received a comprehensive review.
A document that has been accepted by the W3C Director as of sufficient quality to become a W3C Recommendation.
A specification or set of guidelines that, after extensive consensus-building, has received the endorsement of W3C Members and the Director.
Cascading Style Sheets does not have versions in the traditional sense; instead, it has levels.
Each level of CSS builds on the previous level, refining definitions and adding features.
CSS Level 1 (CSS1) became a W3C Recommendation on 17 December 1996. It was revised on 11 January 1999.
The CSS Working Group considers the CSS1 specification to be obsolete.
CSS Level 2 (CSS2) became a W3C Recommendation on 12 May 1998. It was revised on 11 April 2008.
There were many problems with this specification, so instead of trying to fix it, the CSS Working Group decided to release a revised version.
CSS Level 2 Revision 1 (CSS2.1) became a W3C Recommendation on 7 June 2011.
Instead of writing CSS3 as a single specification, the W3C decided to divide CSS3 into a series of individual specifications - called “modules”.
CSS Level 3 builds on CSS Level 2 module by module, using the CSS2.1 specification as its core.
Each module adds functionality and replaces part of the CSS2.1 specification.
This process allows modules to proceed at different rates - rather than wait till all the individual modules are defined.
Despite some of the confusing titles given to various CSS3 modules, there is no such thing as CSS4.
Some of the modules start out at level 3 if they extend something from CSS2.1.
Other modules start out at level 1, if they’re something new, like flexbox.
Regardless, the levels do not correlate to CSS levels. They’re all CSS3.