If you’ve mastered HTML, you’re 90% of the way towards using XHTML. They’re actually very much the same thing—tag-based markup languages used to display Web pages. The difference is only seen by the people creating the pages (Web designers, programmers, etc.) and focuses on “forgivability”— HTML allows for some ugly code (mixed-case tags like <BoDy>, improperly nested elements, and unclosed tags), while XHTML does not. If you write HTML, it’s probably a good idea to start using XHTML practices anyway.
In XHTML, all tags must be lowercase.
In XHTML, all tags must be closed.
In XHTML, tags must be properly nested.
the above are not in html...
And,
* XHTML must be served with an XML MIME type, such as application/xml or application/xhtml+xml.
* HTML must be served as text/html.
It is the MIME type that determines what type of document you are using. If you use attempt to send XHTML as text/html, you are actually just using HTML, possibly with syntax errors.
Technically, according to the spec, XHTML 1.0 is allowed to be served as text/html. But, due to the above reason, such a document is considered to be an HTML document, not an XHTML document.
see it for reference:
http://wiki.whatwg.org/wiki/HTML_vs._XHTML 
Unsolved 
