If you upgrade your site it may have errors in validation. My site had an error and would not validate but it is easy to fix.
the error was on the shipping & returns hyperlink. It should be "Shipping & Returns"
to fix this go to Admin > Content Management > Localization
Serach by pressing ctrl + f and searching for "shipping &" click the edit button and change the Resource value to "Shipping & Returns"
now it will pass validation!