The WordPress 3.5 release can give you this error: Bad value category tag for attribute rel...
The error is given by W3C Validator.
The error can be fixed with these changes: editing file wp-includes/category-template.php
Goto line 163 in your category template (wp-includes/category-template.php) file and change this line of source code.
1 | $rel = ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) ? 'rel="category tag"' : 'rel="category"'; |
and remove the category include whitespace, so that the line looks like this now:
1 | $rel = ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) ? 'rel="tag"' : 'rel="category"'; |
Save the file and check again with W3C Validator.