To get an idea of the differences between XHTML and HTML, let's take a look at the differences between the technologies on which they are based: XML and SGML.
By transforming HTML 4.01 into XML-based XHTML, the W3C hopes to avoid the chaotic evolution that HTML experienced . Both HTML and XHTML use Document Type Definitions (DTDs) to constrain the element and attribute "grammar" of their documents. Adding elements to HTML through new SGML DTDs requires a trip through committees , which is not an easy task. Adding new elements to XHTML is much easier: just add the tags and a namespace.