tag="rel" should not be used in the tag cloud, since those tags do not describe the current page

[DanielStutzbach]

Links to tags tags that describe the current page can be marked with rel="tag", to give search engines an extra clue about what the current page is about. The tags in the tag cloud should not be marked with rel="tag", since they do not necessarily describe the current page (effectively diluting the usefulness of the information).

Quoting the HTML 5 spec:
​http://www.w3.org/TR/html5/history.html#link-type-tag

"Since it indicates that the tag applies to the current document, it would be inappropriate to use this keyword in the markup of a tag cloud, which lists the popular tag across a set of pages"

Also see the older microformat spec:
​http://microformats.org/wiki/rel-tag

Removing the rel="tag" from the tag cloud is a straightforward two-line change in wp_generate_tag_cloud() in category-template.php.

[westi]

(In [12009]) Remove rel='tag' on links in Tag Clouds as it is only for outbound links where the destination is a tag for the current page. Fixes #10897.