The fix suggested by Analogpoint has the downside of making all templates showing up as modified.
Another fix for making vBulletin 3.6.x validate is to hack the includes/functions.php file.
Line 4628 (or somewhere around there) should read:
Code:
$stylevar['textdirection'] = 'ltr';
Replace that with
Code:
$stylevar['textdirection'] = 'ltr" xmlns="http://www.w3.org/1999/xhtml';
and you will have xmlns="http://www.w3.org/1999/xhtml'; inserted in the right place in your <html>-tag.
My suggestion for a fix is much uglier but it has the benefit that it can be undone very easily and doesnt make all your templates show up as modified.
Edit:
After doing some testing I realised that that variable is also used in some of the templates. So there will still be template edits. Hopefully not as many though.