Doing all things as unto the Lord.
There are two primary parts to XHTML:
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 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.
Whereas HTML is an application of Standard Generalized Markup Language (SGML), a very flexible markup language, XHTML is an application of XML, a more restrictive subset of SGML. Because they need to be well-formed, true XHTML documents allow for automated processing to be performed using standard XML tools—unlike HTML, which requires a relatively complex, lenient, and generally custom parser. XHTML can be thought of as the intersection of HTML and XML in many respects, since it is a reformulation of HTML in XML. XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C recommendation on May 31, 2001.