XHTML

XHTML stands for Extensible HyperText Markup Language. It is a reformulation of HTML as an XML application, combining HTML’s document structure with XML syntax rules and tooling. Developed under the World Wide Web Consortium, XHTML was intended to make web documents more rigorous and ensure better interoperability with XML-based technologies, while preserving familiar HTML concepts.

Key features of XHTML are tied to XML well-formedness: documents must be well formed, tags are typically written in lowercase, attribute values must be quoted, elements must be properly nested, and empty elements must be closed (for example, <br />). XHTML documents also use a namespace, commonly xmlns="http://www.w3.org/1999/xhtml", and may include an XML declaration at the top, such as <?xml version="1.0" encoding="UTF-8"?>.

Versions and structure: XHTML 1.0 defined several document types, including Strict, Transitional, and Frameset, each with different levels of presentational markup. XHTML 1.1 introduced a modularized approach to align more closely with XML tooling. The XHTML family also included broader experimental efforts, such as XHTML 2.0, which did not become a standard and was eventually superseded by HTML5-related developments.

Serving and parsing: XHTML can be served as application/xhtml+xml, in which case browsers must parse it as XML and enforce strict well-formedness. If served as text/html, browsers typically treat it as HTML, relaxing XML rules and potentially breaking XML-specific features. In practice, HTML5 has largely absorbed the role XHTML once aimed to fulfill, offering similar capabilities with broader compatibility and simpler parsing for many authors.

Legacy and status: XHTML played a significant role in the early 2000s as a bridge between HTML and XML, but it has been largely supplanted by HTML5 in modern web development.