Stands for "Extensible Hypertext Markup Language." Yes, apparently "Extensible" starts with an "X." XHTML is a spinoff of the hypertext markup language (HTML) used for creating Web pages. It is based on the HTML 4.0 syntax, but has been modified to follow the guidelines of XML, the Extensible Markup Language.
Because XHTML is "extensible," Web developers can create their own objects and tags for each Web page they build. This gives the developers more control over the appearance and organization of their Web pages. The only requirement is that the custom tags and attributes are defined in a document type definition (DTD), that is referenced by the XHTML page.
XHTML pages must also conform to a more strict syntax than regular HTML pages. While Web browsers are rather lenient and forgiving of HTML syntax, XHTML pages must have perfect syntax. This means no missing quotes or incorrect capitalization in the markup language. While the strict syntax requires more meticulous Web page creation, it also ensures Web pages will appear more uniform across different browser platforms.