Getting semantic with Tantek

Tomorrow night, Tantek and I are talking at SDForum about Semantic XHTML:

Can your website be your API? - Using semantic XHTML to make your structures clear

XML formats gained popularity as a backlash against the messiness of HTML mixing structure and presentation, and leniency for sloppy markup. With XHTML+CSS now widely supported in mainstream browsers, and gaining converts even amongst those most focused on representation, these objections lose their force, and the resistance to more and more ad-hoc specialized schemas grows. How far can we get specifying structure in pure XHTML -valid XML - styling it with CSS for presentation, and making it parsable for meaning?