Cartagen
http://cartagen.org/

A vector-based, client-side framework for rendering maps in native HTML 5. Written in JavaScript, it uses the new Canvas element to load mapping data from various sources, including OpenStreetMap. Maps are styled with Geographic Style Sheets (GSS), a cascading stylesheet specification for geospatial information – a decision which leverages literacy in CSS to make map styling more accessible. However, GSS is a scripting language as well, making Cartagen an ideal framework for mapping dynamic data.

A vector-based, client-side framework for rendering maps in native HTML 5

"Just like CSS for styling web pages, GSS is a specification for designing maps. Adapted for dynamic data sources, GSS can define changing geographic elements, display multiple datasets, and even respond to contextual tags like "condition:poor"."