XHTML

XHTML is an XML version of HTML

HTML is already a lot like XML, with some minor textual but important differences:

• XHTML elements must be properly nested
• XHTML elements must always be closed
  E.g. This is incorrect XHTML:
  A break: <br>
  A horizontal rule: <hr>
  An image: <img src="happy.gif" alt="Happy face">
  While this is correct XHTML:
  A break: <br />
  A horizontal rule: <hr />
  An image: <img src="happy.gif" alt="Happy face" />  
  
• XHTML elements must be in lowercase
• XHTML documents must have one root element

See also the W3C HTML and XHTML FAQ.