dofollow link attribute

Great plugin and very useful, thanks.
However, my pages show errors on validation due to ‘dofollow’ being applied to links created by taxonomy link widget. e.g. ‘Bad value dofollow for attribute rel on element a: The string dofollow is not a registered keyword.’

I cannot find a reference for ‘dofollow’ or why it would be needed.
https://moz.com/community/q/need-i-add-rel-dofollow-or-not