When you are on a diff page, the link element for diff.css does not have its type set (Which is required I think).
<link rel="stylesheet" href="/skins-1.5/common/diff.css?156" />
should be something like:
<link rel="stylesheet" href="/skins-1.5/common/diff.css?156" type="text/css" />
Version: unspecified
Severity: trivial