XHTML is a way of writing HTML markup that conforms to the rules of XML.

Despite it being the preferred grammar for EPUB, the XHTML grammar has never gained much traction on the broader web.

Note that although there have been a few versions of XHTML over the years, XHTML is now the name used to refer to the constantly evolving, versionless release of the specification. See HTML5 for further discussion.

XHTML content document is a term used in EPUB 3 to refer to any XHTML document that conforms to the requirements of that format. EPUB 3 has rules about which version of XHTML to use as well as a number of restrictions on the content of the documents.

Refer to the definition of XHTML Content Documents in the EPUB 3 specification for more information.

XHTML 1.1 was a modularization of XHTML 1.0 (e.g., one module defines table markup, another scripting, and so on) with some added features (notably ruby). Modularizing XHTML allowed only parts of the grammar to be used by other standards (e.g., EPUB 2).

XHTML5 is the version of XHTML for HTML5.

Note that as the W3C no longer maintains a versioned HTML specification, XHTML5 and XHTML are often used interchangeably to refer to the latest standard.

The extensible markup language (XML) defines a set of rules for encoding information using elements and attributes.

XML allows specific grammars to be defined for particular uses. A grammar is a specific set of elements and attributes and the rules for how they are structured. Examples of XML grammars include XHTML, SVG, and MathML.