Your theme is close to XHTML 1.0 Strict. Are you considering changing the doctype?
page.tpl.php has to print $head, as explained at http://d-theme.com/grassland, then the doctype could be changed.
WCAG validation is also close. The demo page that I tested has only one conflicting link that passes most tests and fails the Cynthia Says WAI test. The page content contains a link to grasslands at drupal.org and your page heading also contains the same text but in a link back to the page. None of the themes have the page heading, in this case Grasslands, linking back to itself.