Last modified: 2012-04-19 15:09:42 UTC
The Wikimedia projects are not currently passing the W3's markup validation, e.g. see: http://validator.w3.org/check?uri=en.wikipedia.org Currently, 4 errors are reported. The same also applies with CSS, where there are currently 252 bugs on en.wp: http://jigsaw.w3.org/css-validator/validator?uri=en.wikipedia.org (and varying but similar numbers on other projects).
HTML 5 validation fails, too. http://validator.w3.org/check?uri=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMain_Page&charset=%28detect+automatically%29&doctype=HTML5&group=0&user-agent=W3C_Validator%2F1.3