Unfortunately it's unfeasible to get the current DOCTYPE of a theme from the back-end, so it's also impossible to always output W3C-validating XHTML & HTML5 markup.
Take for instance, iframe's XHTML valid
scrolling attribute. It's obsolete in HTML5 and has essentially been replaced by the
seamless attribute (which doesn't work well in modern browsers at the moment FYI). So without knowing the DOCTYPE, it's impossible to print an iframe without scrollbars while maintaining support against W3C's validators.