It appears that the most significant difference between HTML and XHTML is the structure. XHTML, to be properly formatted and validated, is a strict language. HTML is a bit more relaxed and will accept a more flexibly written program. However, XHTML requires a very strictly structured program. For example, XHTML tagging is case sensitive so a tag such as <TD> would not be accepted in this language while HTML would overlook it.




HTML
<img src = john.jpg>
<table WIDTH = 200>
<li>Google
<br>
<div id = class>
<name>
<p>this is good
<hr>
DOCTYPE optional
<input disabled>
XHTML
<img src = “john.jpg” /> must close
<table width = “200”> case sensitive
<li>Google</li> must close tags
<br />
<div id = “class”> must put quotes over attributes
<id>
<p>this is good</p>
<hr />
DOCTYPE Mandatory
<input disabled = “disabled” />



These code snippets change in XHTML:
1. <p>This paragraph must be closed now using this tag
2. <h1>As the woman outran the cat <br> she found that she’s faster.
3. <li>Apples
<li>Oranges
4. <table WIDTH = 20><tr><td>She</td><td>Swan</td><td>Today</td></tr></table>
5. While the dog was eating he <b><i>noticed</b></i> noticed it was cat food.


http://www.w3schools.com/xhtml/xhtml_html.asp
http://wiki.whatwg.org/wiki/HTML_vs._XHTML#Differences_Between_HTML_and_XHTML
http://wdvl.internet.com/Authoring/Languages/XML/XHTML/dif.html