To make you dnn site XHTML compliant, here are the things you should keep in mind:

• XHTML elements must be properly nested
• XHTML documents must be well-formed
• Tag names must be in lowercase
• All XHTML elements must be closed
• Attribute names must be in lower case
• Attribute values must be quoted
• Attribute minimization is forbidden
• The id attribute replaces the name attribute
• The XHTML DTD defines mandatory elements
• Attribute Values Must Be Quoted
• The Lang Attribute. If you use the lang attribute in an element, you must add the xml:lang attribute
• The < !DOCTYPE > Is Mandatory
• Empty Tags: < hr />, < br /> and < img alt="" />. To make your XHTML compatible with today's browsers, you should add an extra space before the "/" symbol like this: < br />, and this: < hr /> .

There is also a blog post on dotnetnuke site that talks about the xhtml compliant.  Click here.