Many Web pages today are poorly written. Syntactically incorrect HTML code may work in most browsers even if it doesn't follow HTML rules. Browsers employ heuristics to deal with these flawed Web pages; however, Web-enabled wireless devices (such as PDAs) can't accommodate these hefty Web browsers. The next step in HTML's evolution comes in the form of XHTML (eXtended Hypertext Markup Language), which is basically a combination of HTML and XML.

XML, the eXtended Markup Language, is a successor for SGML. More general than html, it incorporate data inside tags themselves and has unlimited description capacities. The format of the display is independant, and given by another document, the XSLT. Rules to create tags are defined by another document, the DTD (Document Type Declaration) which describes the grammar of the tags.

Xml features
- Significant tags based upon the content of data.
- Separated document used for the presentation.

Why to use Xml?
This is a standard and universal data format. It allows to reuse a presentation for different data or use different presentations for same data.