Tom,
On the power store page you mention, it actually is not invalid mark-up... what it is has to do with the fact that there are AJAX widgets on the page. This is a known issue that page validators cannot correctly read pages with AJAX and especially Adobe's Spry framework which the page in question does contain three Spry widgets. I know that Adobe has documentation on some additional code that can be placed in the Doctype declaration that tells the W3 Validator not to mistake the Spry code for HTML markup code - which is what happens. The code itself, however, is not invalid - it's just that the W3 Validator thinks it is bad HTML code when it actually has nothing to do with HTML code but rather the Javascript framwework which it calls to perform the tasks on the page.
Warmest regards,
Brian
