Sorry @NewWebDesigner2, didn't see your reply until months later.
XHTML is considered malformed HTML by most browsers unless you also use the correct MIME type, ie application/xhtml+xml. But if you used that in early IE versions you would see an XML DOM, and not a rendered web page. So if you were sending XHTML code, the browser was treating it as HTML anyway. There's no harm in using XHTML, and it's important to use it if you need to process it using an XML parser of some kind (don't forget that XML is strict, compared to HTML), but it wouldn't validate correctly.
See here for more information: http://www.w3.org/TR/xhtml-media-types/#media-types
You'll see that there are acceptable usages of XHTML and text/html (the default for most webservers) but I suspect that they won't cover most of your use cases.
Lots of people think they've been using XHTML for years, but they haven't been doing it properly, frankly.