Double inclusion of class="selected" for the li 's of the namespace/view/actions tables.
See for instance the validator check in the URL
Version: unspecified
Severity: normal
URL: http://validator.w3.org/check?uri=http%3A%2F%2Fen.wikipedia.org%2Fw%2Findex.php%3Ftitle%3DMediaWiki%3AVector.css%26action%3Dedit%26useskin%3Dvector&charset=%28detect+automatically%29&doctype=Inline&ss=1&group=0&user-agent=W3C_Validator%2F1.654#line-122