Use XHTML string in EPiServer to handle XHTML correctly
If you anytime bump into the problem with XHTML code which isn't interpreted correctly in EPiServer, you might want to take a look at what EPiServer property type contains the XHTML code.
If you use the common Long string property type, the XHTML will be treated as HTML and the code will be shown in uppercase, thus not validating.
Instead, you should use the XHTML string property type.
If you've got any problems, just change the property type to XHTML string (<255) and your problems will probably be solved.