We mentioned recently Jeroen Sangers's discovery that the latest version of Movable Type generates invalid markup. It's been fixed (via Matt Burris).
Movable Type developer (and Six Apart co-founder) Ben Trott:
Update: We've released 2.661, an update to 2.66, to fix a problem with invalid XHTML in the comment redirect script, along with using comment IDs instead of URLs to fix an open redirect problem.
Note that we're not sure when this update was released since the fine folks at Movable Type aren't using the ins X/HTML element, a subtle semantic enhancement, which, when used with the datetime attribute, not only alerts users to additional content, but more important, indicates when the content was added.