The extensible hypertext markup language (XHTML) is a quick way to refer to several language recommendations that are widely used on Internet-enabled devices for viewing web pages. Although named after its predecessor, the hypertext markup language (HTML), it's is actually based on the extensible markup language (XML), which is a very selective part of the standard generalized markup language (SGML). In essence, they're all offspring of SGML. While HTML is a direct application of SGML, XHTML is what's referred to as a namespace, or a set of definitions for an XML document that helps to relieve ambiguity when more than one XML vocabulary is being used in any given situation.
The language came about because of a few limitations to HTML and the varied way HTML was being implemented. Around the time HTML made it to version four, it began to wane in proper usage by many HTML interpreters, the computer programs that parse HTML documents into a formatted, viewable web page. As mobile devices and other web-viewing platforms were also emerging, a better solution was needed. XML is a much more strict implementation of SGML over HTML, and different XML namespaces can be used in a single instance. So around the year 2000, the World Wide Web Consortium (W3C) drafted and made XHTML one of its recommendations to solve some of these emerging issues.
For all intents and purposes, XHTML mimics HTML in most ways, but since the former uses an XML namespace, it can be parsed by any XML interpreter, while HTML is limited to only HTML interpreters. XHTML is really HTML recreated under the more restrictive XML subset of SGML. In this way, the more recent language was immediately able to be interpreted by existing web browsers while also making itself available for other platforms. Living up to the extensible aspect of XHTML's moniker is also important to note. It not only offers the ability to be read by more programs and platforms, but it is also further extensible by allowing the use of other XML namespaces within its documents.
With XHTML's ability to include other XML namespaces in a document, it can be extended in a number of ways to present more than just page formatting. The mathematical markup language (MathML), for example, can be included in these documents to display mathematical formulas and notation. Images can also be embedded using the scalable vector graphics (SVG) namespace within a document of this type. As such, XHTML can also be a included with another XML document.
Since XHTML is really just HTML refined under XML's rules, it offers three document type definitions (DTD) that duplicate those of HTML version four. A DTD is a detailed description of the elements of a markup language, including when, where, and how it can be used, as well as any associated attributes. In later versions of XHTML, however, XML schemas, another, more robust way of describing an XML document, were established that further augmented XHTML. In turn, various stripped-down versions of XHTML were developed that can then be built upon for specific uses, many of which revolve around mobile computing platforms.