XHTML

XHTML (Extensible HyperText Markup Language) is a family of XML-based reformulations of HTML. The first version, XHTML 1.0, became a W3C Recommendation in January 2000 and was essentially HTML 4.01 expressed under XML syntax rules: lowercase tag and attribute names, quoted attribute values, explicit closing of every element, and no implicit content models. XHTML 1.1, published in 2001, completed the migration by formally dropping deprecated presentational features and modularizing the language. The motivation for XHTML was twofold. XML was expected to become the lingua franca of structured data exchange, and recasting HTML under XML promised a single toolchain — XML parsers, XSLT, XPath, namespaces — for both documents and data. It also imposed discipline on a language whose sloppiness was widely blamed for browser interoperability problems. In practice the migration ran into a structural problem. XHTML inherited XML's draconian error handling rule: a single well-formedness error must abort parsing and produce an error rather than a rendered page. Most authors never delivered XHTML with the proper ``application/xhtml+xml`` MIME type, instead serving it as ``text/html`` where browsers continued to use forgiving HTML parsers. As a result the XML strictness existed mostly on paper. The XHTML 2.0 working group attempted a more radical redesign starting in 2002, dropping backwards compatibility entirely and reaching for cleaner abstractions: any element as a link, generic headings, XForms instead of HTML forms. That effort was abandoned in July 2009 when its W3C charter was allowed to expire. The work was superseded by HTML5, which kept HTML's forgiving text serialization while also defining XHTML5 — an XML serialization of the same content model. XHTML survives today primarily as that serialization choice rather than as a separate language.