CSS is the abbreviation for Cascading Style Sheets and is a stylesheet language that is used for the purposes of describing a document that was written in a mark-up language. This is a set or group of annotations to text that describes certain aspects such as format, layout, and structure. Ironically enough, mark-up languages are not new and have been used for centuries. Recent use of mark-up languages includes computer typesetting and word processing systems.
The most common application of CSS is with webpages on the internet that have been written in code --- specifically HTML or XHTML. The language has applications to any type of XML document, SVG and XUL inclusive. CSS also helps readers of web pages to define certain facets such as color, fonts, layout, and other forms of the document's presentation.
It was to designed to enable the separation of the document's content when written in HTML, from the presentation of the document when it was written in CSS. Content accessibility is generally improved, but more flexibility results as well as control in presentation characteristics specifications. This results in the reduction of the complexity and repetition of the structural content.
CSS also allows presentation of the specified page in different styles and rendering methods. This would include on-screen, print, and voice applications, as well as Braille-based, tactile devices. If there is a situation where more than one rule applies, CSS can specify a priority scheme that can determine which rules apply. Results become predictable because the "cascade", priorities, or weights are calculated and assigned to the applicable rules. Specifications for CSS are mandated by the World Wide Wed Consortium (W3C) Internet media type (MME type).
CSS uses a simple syntax that employs a number of keywords written in English in order to specify the names or descriptions of assorted style properties. Style sheets are comprised by a list of rules and the individual rules (or sets of rules) are comprised of single or multiple selectors and what is called a declaration block. Declaration blocks are characterized by a list or lists of semi-colon separated declarations that are contained in curly braces.
The elements that a style applies to are declared by what is called selectors and is more like an expression that denotes a match. Selectors can apply in one of two ways --- they can apply to all the elements of a specific type, or they may refer to any and all elements that match certain attributes. The way elements are placed within a mark-up code can determine the way in which they are matched.
Prior to the onset of CSS, nearly all of the HTML presentational attributes within the document were found within the HTML mark-up. Background styles, borders, element alignments, font colors, and sizes were described almost repetitiously within the HTML code. Simpler HTML mark-up can result because CSS allows the author to move the information to another style sheet.
For more information on CSS and to find tutorials regarding the subject, visit http://www.w3.org/Style/CSS.