The "X" in XHTML signifies HTML's addition to a large family of extensible, interoperable, and self-describing markup languages. This is the Extensible Markup Language (XML) family, which defines a general way to create application-specific markup languages for different purposes. The XML clan frequently welcomes new members, including XHTML, Scalable Vector Graphics (SVG), and Rich Site Summary (RSS) for news syndication. In this chapter you'll learn how to push XHTML to extremes, but abide by the family rules.
Due to its stricter rules, XHTML can use a subset of the optimization techniques you learned for HTML.