W3 validation: a disaster!

To begin with, somehow the DOCTYPE was removed from your header.php file – which causes major problems in IE. So start over in your child theme with a new copy of the header.php file and whatever changes you make to it, don’t remove or change existing code. Most of your other errors are probably not causing problems, but this gives you some additional information:

http://codex.wordpress.org/Validating_a_Website