Links widget fails W3C Validation
My new test blog, Layman's Bench, uses the Kubrick theme. With the default sidebar, Kubrick displays two links (WordPress.com and WordPress.org under the heading "Blogroll") and does indeed validate as XHTML 1.0 Transitional per the W3C Validator.
But if one goes to the Sidebar Widgets page, does a "drag and drop" putting the Links widget into the Sidebar box, and saves the change, the new rendition of Kubrick fails validation. It seems there's a list item tag (<li>) associated with the Category title ("Blogroll" in this case) which in not enclosed in any list structure (such as <ul>).
Deactivate the Links widget (and all others) to return to the default sidebar, and the page validates.
Based on my limited and random experimentation, inclusion of the Links widget is all it takes to fail validation. None of the other widgets, combinations of widgets, or the default sidebar failed validation.
I have set up a demonstration at Layman's Bench which I shall try to leave in place for a week or so, or until I hear that the "powers that be" have seen it.