HTML was originally designed for publishing hypertext on the World Wide Web. HTML is a non-proprietary
format based on SGML. Variations in HTML language definitions have emerged in the form of browser specific implementations. These variations, combined with the increasing availability and diverse capabilities of non-desktop browsers, such as PDAs and mobile phones, have made it difficult for designers to settle on a system for targeting a range of devices. Many developers found themselves having to take a "lowest common denominator" approach when trying to combine content, physical layout and interactive behavior of a web application designed for a broad spectrum of client devices.
XHTML (Extensible Hypertext Mark-up Language), created by the World Wide Web Consortium W3C,
addresses this issue. XHTML takes advantage of the more powerful meta-language XML and thus requires
that documents be "well-formed". Variations are not allowed. The strict mark-up standards are what allow
extensibility of the core tag set to extend functionality.
XHTML 1.0 has replaced HTML as the official Web mark-up standard according to the W3C. XHTML is also the content authoring language specified by WAP 2.0 and XHTML Mobile Profile, used by many handheld devices, is a subset of XHTML.
XHTML looks a lot like HTML but has more rigid mark-up standards. XHTML documents must strictly
conform to these standards, that is, be "well-formed". For more on these standards and how to convert
existing HTML to XHTML, see working with XHTML in this manual.