CSS – Cascading Style Sheets
CSS, developed in 1997, defines the presentation semantics to display HTML elements. CSS is designed primarily to enable the separation of document content from presentation elements such as the layout, fonts and colors. Style sheets are simply text files with ".css" extension.
CSS is supported by all browsers today and has become a powerful tool for Web designer to change the mood and tone of a Web site. The term cascading derives from the fact that multiple style sheets can be applied to the same Web page.