Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation semantics (the look and formatting) of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can also be applied to any kind of XML document, including plain XML, SVG and XUL.

Sources
CSS information can be provided by various sources. CSS style information can be either attached as a separate document or embedded in the HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so that authors can tailor the presentation appropriately for each medium.
Priority scheme for CSS sources (from highest to lowest priority):

• Author styles (provided by the web page author), in the form of:
• Inline styles, inside the HTML document, style information on a single element, specified using the “style” attribute
• Embedded style, blocks of CSS information inside the HTML itself
• External style sheets, i.e., a separate CSS file referenced from the document
• User style:
• A local CSS file the user specifies with a browser option, which acts as an override applied to all documents
• User agent style
• Default styles applied by the user agent, i.e., the browser’s default settings for element presentation

The style sheet with the highest priority controls the content display. Declarations not set in the highest priority source are passed on by a source of lower priority such as the user agent style. This process is called cascading.
One of the goals of CSS is also to allow users greater control over presentation. Someone who finds red italic headings difficult to read may apply a different style sheet. Depending on their browser and the web site, a user may choose from various style sheets provided by the designers, may remove all added style and view the site using the browser’s default styling, or may override just the red italic heading style without altering other attributes.