Notwithstanding the 'potential' problems with sending xhtml as text/html - see: http://www.hixie.ch/advocacy/xhtml (or google for similar documents) - the idea is to encourage the creation of valid markup - and have all markup passed through a validator first (which eliminates the 'real' problems).

On that note - it might be a good idea to include links to pages like: http://www.hixie.ch/advocacy/xhtml part of the best practices documentation.