The document discusses the history and evolution of CSS (Cascading Style Sheets). It explains that CSS1 was introduced in 1996 and offered basic formatting capabilities. CSS2 was released in 1998 and introduced additional features like positioning. CSS3 has been divided into modules since work began on it in 1999, with each module adding new capabilities or extending CSS2 features while maintaining backward compatibility. The document then discusses some important CSS3 modules and the browser support for CSS3.