What is CSS?
The first CSS specification to become an official W3C Recommendation is CSS level 1, published on December 17, 1996. Håkon Wium Lie and Bert Bos are credited as the original developers.Among its capabilities are support for
CSS level 2 specification was developed by the W3C and published as a recommendation in May 1998. A superset of CSS 1, CSS 2 includes a number of new capabilities like absolute, relative, and fixed positioning of elements and z-index, the concept of media types, support for aural style sheets (which were later replaced by the CSS 3 speech modules) and bidirectional text, and new font properties such as shadows.
The W3C no longer maintains the CSS 2 recommendation.
CSS level 2 revision 1, often referred to as "CSS 2.1", fixes errors in CSS 2, removes poorly supported or not fully interoperable features and adds already implemented brow py-2ser extensions to the specification. To comply with the W3C Process for standardizing technical specifications, CSS 2.1 went back and forth between Working Draft status and Candidate Recommendation status for many years. CSS 2.1 first became a Candidate Recommendation on February 25, 2004, but it was reverted to a Working Draft on June 13, 2005 for further review. It returned to Candidate Recommendation on 19 July 2007 and then updated twice in 2009. However, because changes and clarifications were made, it again went back to Last Call Working Draft on 7 December 2010.
CSS 2.1 went to Proposed Recommendation on 12 April 2011. After being reviewed by the W3C Advisory Committee, it was finally published as a W3C Recommendation on 7 June 2011.
CSS 2.1 was planned as the first and final revision of level 2—but low priority work on CSS 2.2 began in 2015.
Unlike CSS 2, which is a large single specification defining various features, CSS 3 is divided into several separate documents called "modules". Each module adds new capabilities or extends features defined in CSS 2, preserving backward compatibility. Work on CSS level 3 started around the time of publication of the original CSS 2 recommendation. The earliest CSS 3 drafts were published in June 1999.
Due to the modularization, different modules have different stability and statuses.
Some modules have Candidate Recommendation (CR) status and are considered moderately stable. At CR stage, implementations are advised to drop vendor prefixes.
There is no single, integrated CSS4 specification, because it is split into separate "level 4" modules.
Because CSS3 split the CSS language's definition into modules, the modules have been allowed to level independently. Most modules are level 3—they build on things from CSS 2.1. A few level-4 modules exist (such as Image Values, Backgrounds & Borders, or Selectors), which build on the functionality of a preceding level-3 module. Other modules defining entirely new functionality, such as Flexbox, have been designated as "level 1".
The CSS Working Group sometimes publishes "Snapshots", a collection of whole modules and parts of other drafts that are considered stable, interoperably implemented and hence ready to use. So far, five such "best current practices" documents have been published as Notes, in 2007, 2010,2015, 2017, and 2018.