HTML Versus XHTML: Which Should We Use, and Why?
http://www.webstandards.org/learn/articles/askw3c/oct2003/

Web Standards Project
2005

Abstract:

HTML 4.01 is as valuable as XHTML 1.0 in a daily usage. The syntax proposed by XHTML 1.0 has several important benefits. The weight of these benefits has to be evaluated in the context of your project: Use the right tool for the right job. For a Web designer, starting to use XHTML 1.0 will be helpful in some circumstances and will certainly help you to smoothly negotiate the future. XHTML 1.0 gives a wonderful opportunity to learn about XML languages and their possibilities without having to learn new semantics because you’re working with familiar tags and attributes.