FILE: /skins/[skin name]/customer/common_templates/product_data.tpl
You will notice that this form tag will have 2 declared class attributes if “ajax_add_to_cart” is true:
This will result in W3C error. The IF statement should be placed within class attribute, and "cm-disable-empty-files" should be moved into the first class attribute:
In this case, both classes will be set in one class attribute:
Like I've said, nothing major, but still worth looking into fixing for future releases.;)
Peace and blessings!