Check the doctype validity using W3c validator (hml , xhtml, ...) when you create or update your page / post / custom post type and show the result in
It's a great idea, but has some problems.
First of all, it will give you no warning if you've just posted/edited html that doesn't validate. It only shows on the posts listing.
Second, it's not completely translated to English. For example, Enregistrer les modifications should be "Save Setting" in English.