HTML is well-known as a mark up language. However, XML is not as well known. XHTML is the cross section between the two. It makes websites have more depth to their programming capacity.
The main problem with XHTML is validity. Making a website is easy; making a valid website, not so easy. The tricky thing about validity problems is that they're hard to detect. A website may still be viewable, but it may have portions that are broken, or a person may be able to look at it, but search engines might just toss it aside. The worst situation for an invalid site is when search engines accept and then index it because when a person looks at it, it may look ugly.
Situations like the above happen because programmers don’t test their sites against several different browsers. Contact us, and we’ll make sure your website is made compatible.