XHTML

XHTML. The Extensible HyperText Markup Language is a family of XML-based markup languages that mirror or extend versions of HTML. It was developed by the World Wide Web Consortium as a stricter, more structured reformulation of HTML to be compatible with XML-based tools and to promote cleaner, more interoperable web code. The transition aimed to bridge the gap between the lenient syntax of classic HTML and the rigorous, well-formed requirements of the XML specification.

History and development

The development of XHTML was driven by the World Wide Web Consortium during the late 1990s, as the web expanded beyond the initial design of HTML. Key figures and working groups within the W3C, influenced by the success of XML, sought to redefine HTML as an XML application. This effort culminated in the first W3C Recommendation for XHTML 1.0 in January 2000. Subsequent versions included XHTML 1.1, which introduced modularization, and the ambitious XHTML 2.0 project, which ultimately stalled due to its lack of backward compatibility with existing HTML content. The failure of XHTML 2.0 led to a shift in focus by the W3C and competing groups like the Web Hypertext Application Technology Working Group, resulting in the development of HTML5, which reintegrated many XHTML-inspired syntactic rules as an optional XML serialization.

Relationship to HTML

XHTML maintains a close but complex relationship with its predecessor, HTML. Fundamentally, XHTML is HTML reformulated as an application of XML, meaning it must adhere to the stricter syntax rules defined in the XML 1.0 specification. While HTML, as defined by RFC 1866 and later specifications, allowed for forgiving parsers and tag soup, XHTML requires well-formedness, with all elements properly nested and closed. This relationship was intended to make web documents processable by standard XML parsers like those used for SVG or MathML, enabling greater integration within the broader XML ecosystem. The HTML5 specification later formalized this duality by defining two serializations: the classic HTML syntax and the XHTML5 syntax, which is HTML5 expressed as valid XML.

Syntax and structure

The syntax of XHTML imposes several key structural requirements derived from the XML specification. All element and attribute names must be written in lowercase, a departure from case-insensitive HTML. Every start tag, such as `

` or ``, must have a corresponding end tag or be self-closing, and all attribute values must be quoted within either single or double quotation marks. Documents must begin with a proper XML declaration or a Document Type Definition declaration, such as the one for XHTML 1.0 Strict. Furthermore, the nesting of elements like `