Rel = Category Tag

The W3C is no longer up to date and these errors should be ignored.
If you try and validate other WordPress, you’ll find the same thing.

The rel="category tag" is part of the rel-category microformat specification:
https://microformats.org/wiki/rel-category

Note that the W3C validator tools are meant as guides, not to be taken as absolute truths. Their checks may not be up to date with the current standards that are in widespread use.

Should it not be category_tag?

While this statement is highly debatable, some will argue that the “correct” (and I use the term very loosely) rel attribute should be tag. I’m not going to debate the validity of HTML validation but if you prefer you can use this filter I wrote that addresses the “issue”

/**
  * Filters the_category() to output html 5 valid rel tag
  *
  * @param string $text
  * @return string
  */
function html_validate( $text ) {
	$string = 'rel="tag"';
	$replace = 'rel="category tag"';
	$text = str_replace( $replace, $string, $text );

	return $text;
}
add_filter( 'the_category', 'html_validate' );

ahh many thanks

Ok have put this filter in and it works great thanks…….

Thanks for your help I am obliged.