Another child of SGML is Extensible Markup Language (XML). Interestingly, XML is also a meta-language and a means of creating other languages. While SGML is very complex and extremely detailed, XML has emerged as a streamlined meta-language suitable for creating web markup languages that are customizable and flexible for the needs of specific applications. Examples of XML markup applications include Scalable Vector Graphics (SVG), Synchronized Multimedia Integration Language (SMIL), and Wireless Markup Language (WML).
As stewards of the evolution of markup languages, the W3C began to look at HTML and the problems it was facing due to this stretching and manipulating to accommodate design. Viewed through the lens of XML, HTML is clearly a linguistic markup mess. So the W3C applied the strength and logic of XML to the best of HTML. From this work came a new, refined markup language, the Extensible Hypertext Markup Language (XHTML).
XHTML is the reformulation of HTML as an XML application. In other words, the rules and methodologies of XML are applied to HTML, bringing syntactical strength back into HTML, which lost that strength during its rapid evolution from text document markup language to the de facto language of visual design. By strengthening markup in this way, markup is brought much closer to the interoperable, accessible, international, and growth-oriented goals of all designers.