Differences Between XHTML, HTML, and XML

XHTML is HTML 4.0 Written in XML Format
On January 26, 2000, the W3C announced that HTML 4 has been reformulated into XHTML 1.0. XHTML is HTML 4 written as an XML application.

There are two primary parts to XHTML:

HTML 4
HTML 4 is a markup language used for displaying text and documents across different platforms and machines. It was originally intended for a very specific audience, and has expanded to include hypertext, multimedia, as well as the style of the documents displayed.

XML
XML is an extensible markup language that was developed to retain the flexibility and power of HTML while reducing most of the complexity.

XHTML combines the flexibility of HTML with the extensibility of XML.

Benefits of XHTML

A painless transition to more advanced technology

The web is moving to XML, a powerfully enabling technology. Writing well–formed, valid XHTML pages is the easiest way to begin this transition. All it takes is learning a few simple rules of XHTML markup.
Cleaner, more logical markup
XHTML brings uniformity to document structure. The rules of XHTML help restore the structural integrity of documents that was lost during the web’s rapid commercial expansion between 1994 and 2001. This is critical for large organizations such as ours, whose web pages must interface with logically–marked–up documents in legacy systems and databases.

Increased interoperability
Unlike old–style HTML pages, valid, well–formed XHTML documents can easily be “transported” to wireless devices, Braille readers and other specialized web environments. Moreover, XHTML’s insistence on clean, rule–based markup helps us avoid the kind of errors that can make web pages fail even in traditional browsers like Microsoft Internet Explorer, Netscape Navigator, and Opera Software’s Opera browser.

Greater accessibility
Because they follow strict rules and avoid non–standard markup, well–authored XHTML pages are more accessible than old–school HTML pages, helping the library comply with U.S. laws and accessibility guidelines.