W3C:

2008-05-23: The Cascading Style Sheets (CSS) Working Group has published the Candidate Recommendation of CSS Namespaces Module. This CSS Namespaces module defines the syntax for using namespaces in CSS. It defines the @namespace rule for declaring the default namespace and binding namespaces to namespace prefixes, and it also defines a syntax that other specifications can adopt for using those prefixes in namespace-qualified names. Learn more about the Style Activity. (Permalink)