Who said that? It's not because (most) browsers support it, that you should use it... What is wrong with HTML? If you don't have a use for XHTML then why use it? XHTML is an extention to HTML... BTW, most pages I encountered on the web that claim to be XHTML don't even have a DTD specification and are just normal HTML pages (but with the error like /> in them)... So don't mix up standards
To give examples:
For non-empty elements, end tags are required in XHTML. This means you can't use:
<p>this is a line<p>this is a new line.
but you must use:
<p>this is a line</p><p>this is a new line.</p>
This is not required in HTML. Now look at the source of this thread... It claims to be XHTML, but yet <p> is used without </p>.
Attribute values must always be quoted in XHTML. That's something many people forget. You can't use:
<img with=100 height=100 src=http://blahblah>
but you must use:
<img with="100" height="100" src="http://blahblah">
This is not required in HTML.
My whole point is, don't use XHTML if you don't need it. You don't use Flash either if you don't have a use for it