Cascading Style Sheets describe how documents are presented on screen in the browser. CSS separates the presentation from the content. Changes to the presentation can be made in the style sheet and the changes are automatically reflected throughout the whole document.
CSS is critical for the mobile browser by making it easy to display the same document on wide variety of devices, each with its own display formats and capabilities. The power of CSS lies in the precise control if offers to document creators and the ease with which they can create content for presentation on any device. Every aspect of the document (positioning, fonts, text attributes, borders etc) can be defined in the style sheet. A change to any aspect of the document is needed to make only once.