By using CSS, presentation is separated from the structure or content of the document. On such documents, the presentational attributes can be provided on a separate style sheet which results in a simpler HTML markup. By using CSS, a single HTML document can be presented in different styles using different methods.

When talking about style sheets, the term refers to the document itself. Style sheets have been used to provide the technical specifications for a layout. Using a style sheet ensures that the specifications of the author or designer is applied for a certain document. Every web page makes use of at least one style sheet.

Even if there are no instructions provided on how a certain document is presented, the web browser makes use of default style sheets to display a web page. The "Cascading" in CSS refers to the set of instructions that takes precedence over the other rules set if there is more than one set of instructions is found on one document.

By using CSS, web designers can develop a websites entire mood and tone easier. With using well written style sheets, web pages can easily be updated without affecting the underlying XHTML document or structure.

CSS has become a required tool for most web designers today. It may be a bit difficult to understand at first than plain HTML. But once the designer gets used to it, it can be a powerful tool to use in designing web sites.