Even for an empty page, the generated XHTML does not pass validation by validator.w3.org (via the button at the bottom of the page). I get 5 errors and 1 warning:
Warning:
DOCTYPE Override in effect!
Errors:
Line 2, Column 33: there is no attribute "class"
Line 2, Column 1: Missing xmlns attribute for element html. The value should be:
http://www.w3.org/1999/xhtml
-> <html lang="de" dir="ltr" class="no-js">
Line 4, Column 19: there is no attribute "charset"
Line 4, Column 28: required attribute "content" not specified
-> <meta charset="utf-8" />
Line 7, Column 12: required attribute "type" not specified
-> <script>(function(H){H.className=H.className.replace(/\bno-js\b/,'js')})(do…
I couldn't care less about validation to be honest, but since the tooltip for the validator button says "Valid XHTML 1.0", it probably should be just that.