Check the doctype validity using W3c validator (html , xhtml , … ) when creating or updating page / post / custom post type and show the result in backend It show the result in back-end in sortable column, a link is add to the w3c for all file to help you to correct html error.
Very easy to see if a post / page is valid or not.
This plugin can check the W3C validity of your page / post or custom post type even if the site is not accessible from the Internet (if you work on local system for example)