Confront:

http://validator.w3.org/check?uri=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMain_Page&charset=%28detect+automatically%29&doctype=Inline&group=0&user-agent=W3C_Validator%2F1.1

Removing the <ul>s would eliminate the error. For logged-in users, the <ul> contains the top bar entries - links to the userpage, usertalk, sandbox, preferences and whatsnot. The code which decides what to display between <ul> and </ul> should decide instead not to display anything when this tag is empty.

Version: 1.16.x
Severity: minor