Software Development Engineer Question:

CSS (Cascading Style Sheets) is a technical specification that allows HTML document authors to attach formatting style sheets to HTML documents. When HTML documents are viewed as Web pages through Web browsers, the attached style sheets will alter the default style sheets embedded in browsers.

One of the fundamental features of CSS is that style sheets cascade; authors can attach a preferred style sheet, while the reader may have a personal style sheet to adjust for human or technological handicaps. The rules for resolving conflicts between different style sheets are defined in CSS specification.

CSS specification is maintained by W3C. You can download a copy of the specification at http://www.w3.org/.
Tutorials below are based Cascading Style Sheets, level 1, which has been widely accepted as the current standard.