"The border attribute on table only allows the values '1' and the empty string." in HTML5 [1]
On [[Special:Prefixindex]] there are 2 tables with border="0": id="mw-prefixindex-nav-table" and id="mw-prefixindex-list-table". I think that it just can be deleted, no border is the default, and [[Special:AllPages]] doesn't use this attribute, too.
[1] http://www.w3.org/TR/html5-diff/#changed-attributes
Version: unspecified
Severity: trivial
URL: http://www.w3.org/TR/html5-diff/#changed-attributes