XML (Extensible Markup Language) is structured set of rules for how to define any kind
of data that has to be shared on the Web. It's called "extensible" because anyone can invent a particular
set of markup for a particular purpose and as long as everyone uses it,can be adopted and used for different purposes -
including, as it happens, describing the appearance of a Web page.
However, the immediate issue is to facilitate transition from HTML for the mass of developers
who are already familiar with HTML. That being the case, it seemed desirable to reframe HTML in the terms of
XML. The result is XHTML, which is a particular application of XML for "expressing" Web pages.
XHTML is the follow-on version of HTML 4. You could think it as HTML 5, except that
it is called XHTML 1.0. In XHTML, all HTML 4 markup tags and attributes
will continue to be supported.
With HTML, authors had a fixed set of elements to use, with no variation. Unlike HTML, however,
XHTML can be extended by anyone who uses it. New tags and attributes can be defined and also added to
those that are already existing, making possible new ways possible to embed content and programming in Web page.
XHTML 1.0, allow authors to mix and match the known elements of HTML 4 elements with that of other XML languages elements.
languages, including the one which are developed by W3C for multimedia.
We combine HTML with other tag sets to meet the desires to extend the functionality of the web.
(Synchronized Multimedia Integration Language - SMIL), mathematical expressions (MathML), two
dimensional vector graphics (Scalable Vector Graphics - SVG), and metadata (Resource Description
Framework - RDF).