This document discusses HTML, XHTML, and CSS. It defines HTML as the code that defines web page structure, and XHTML as a stricter version of HTML that follows XML rules. CSS is described as the code that controls how web pages are displayed and structured. The document then provides terminology for key concepts in XHTML and CSS, such as tags, attributes, elements, declarations, properties and values. It notes the advantages of combining XHTML and CSS include separating structure from presentation for easier updates and redesigns.