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).

The reasons to upgrade language to a new version is to take advantage of new bells and whistles, and also because problems with the earlier version have been fixed. However, XHTML is just a faithful copy of HTML 4, as tag functionalities go, so here we cant expect any fancy new tags.

According to W3C XHTML should be used because of:

• Extensibility

  XML documents needs to be well-formed. Under HTML (an SGML application), any addition of a new group of elements requires to alter the entire DTD. In an XML-based DTD, all that is required is that the new set of elements be internally consistent and well-formed to be added to an existing DTD. This makes the development and integration of new collections of elements very easy.

• Portability

  Now days use of non-desktop devices to access Internet documents is increasing .75% of Internet access could is carried out on these alternate platforms. most of these non-desktop devices will not have the computing power of a desktop computer, they are not designed to hold ill-formed HTML as current browsers tend to do.These non-desktop browsers will not display the document if they wont receive well-formed markup (HTML or XHTML).

While HTML isn't completely lacking those attributes, we're all too familiar with how painfully slow the evolution has been (relative to the pace of Internet development), and how hard it it is to make your pages work on a wide range of browsers and platforms. XHTML will help to remedy those problems.