When I tried to validate my site in HTML through I got this error in my blog homepage.
Bad value category tag for attribute rel on element a: Keyword category is not registered.
I tried to find its solution but nothing helped. Then after so many efforts, I finally found the solution.. Just add the following in your functions.php file and see the magic…

Your error has been validated!!

add_filter( 'the_category', 'add_nofollow_cat' ); 
function add_nofollow_cat( $text ) { 
$text = str_replace('rel="category tag"', "", $text); return $text;