XHTML Conformance

Must obey the XHTML DTDs.
Root element must be html.
The html must contain an xmlns pointing to the the XHTML namespace.
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
A DOCTYPE must appear before the html and point to the appropriate XHTML DTD.
Cannot override any parameter entities in the DTD.

Example:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html 
     PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
    <title>Virtual Library</title>
  </head>
  <body>
    <p>Moved to <a href="http://example.org/">example.org</a>.</p>
  </body>
</html>