I get an SGML error (tidy) on each page that says:
duplicate specification of attribute "class"

It is caused by by this element:
<li id="ca-edit" class="selected" class="selected">

(to be more exact - by any selected element in the div#views; that is in upper, right navigation bar)

Version: unspecified
Severity: normal
URL: http://usability.wikimedia.org/wiki/Main_Page#News