CSS3 is the latest evolution of the Cascading Style Sheets language and aims at extending CSS2.1. It brings a lot of long-awaited novelties, like rounded corners, shadows, gradients, transitions or animations, as well as new layouts like multi-columns, flexible box or grid layouts. Experimental parts are vendor-prefixed and should either be avoided in production environments, or used with extreme caution as both their syntax and semantics can change in the future.
Modules and the standardization process
CSS Level 2 needed 9 years, from August 2002 to June 2011 to reach the Recommendation status. This was due to the fact that a few secondary features hold back the whole specification. In order to accelerate the standardization of non-problematic features, the CSS Working Group of the W3C, in a decision referred as the Beijing doctrine, divided CSS in smaller components called modules . Each of these modules is now an independent part of the language and moves towards standardization at its own pace. While some modules are already W3C Recommendations, other still are early Working Drafts. New modules are also added when new needs are identified.
Formally, there is no CSS3 standard per se . Each module being standardized independently, the standard CSS consists of CSS2.1 amended and extended by the completed modules, not necessary all with the same level number. At each point of time, a snapshot of the CSS standard can be defined, listing CSS2.1 and the mature modules.
Though today no module with a level greater than 3 is standardized, this will change in the future. Some modules, like Selectors 4 or CSS Borders and Backgrounds Level 4 already have an Editor’s Draft, though they haven’t yet reached the First Published Working Draft status.
CSS modules status
A few CSS modules are already fairly stable and have reached one of the three recommendation level of the CSSWG: Candidate Recommendation, Proposed Recommendation or Recommendation. These can be used without prefix and are pretty stable, though a few features can still be dropped at the Candidate Recommendation stage.
These modules extend and amend the CSS2.1 specification which build the core of the specification. Together with it, they are the current snapshot of the CSS specification.