You probably already found an answer, but I learned from the Softpress support team that Exhibio will work okay even if the page isn’t set to HTML 5. It might not “validate” entirely properly, but it should work – at least in XHTML transitional, which is what my query was about. And it seemed to.
(Ran into another issue, which may be a conflict with ScriptyLightBox 3 running on the same page. Seemed to make the slideshow stop. Trying to learn some resolution on that now.)