WordPress has a handy little function called the_category() which outputs a link to the current post category in the loop. Unfortunately it adds a rel tag that breaks HTML5 validation. Here’s a quick fix for your theme’s functions.php.

To stop WordPress from adding rel="category tag" to your category links, just edit your theme’s functions.php file and add the following code:

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add_filter('the_category', 'remove_category_rel');
function remove_category_rel($string)
{
    return str_replace('rel="category tag"', '', $string);
}

The above function just replaces the rel attribute with… nothing. Proceed with HTML5 validation.