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 semantic 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.

CSS Modules and Snapshots as defined since CSS3 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.

The W3 consortium periodically publishes such snapshots, like in 2007 or 2010.

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 reach the First Published Working Draft status.