Potentially bad value X for attribute “rel” on element “a” Typo for Y?

The rel attribute on an <a> element contains a token that looks like a misspelled link type, such as _nofollow where nofollow was meant.

rel holds a space-separated list of keywords that describe how the current page relates to the linked resource. Browsers and search engines only act on the keywords they recognize, and they compare each token as written, ignoring case. A near miss such as _nofollow, nofolow or no-follow is not read as nofollow, so search engines see an ordinary followed link. A misspelled noreferer has a similar effect: the browser still sends the Referer header to the destination. The HTTP header is spelled with one "r", but the keyword is noreferrer.

The validator does not report every unknown token, because rel also accepts extension values defined outside the HTML standard. It warns only when a token is close to a known keyword, and the message ends with the keyword it suspects you meant. When the attribute holds several tokens, the message quotes the whole value, so check each token to find the misspelled one.

A leading underscore is a frequent cause. Keywords for the target attribute start with one (_blank, _self), and the habit carries over to rel on the same link. No rel keyword starts with an underscore.

Invalid example

<a href="https://example.com/" target="_blank" rel="_nofollow">Partner site</a>
<a href="https://example.org/" rel="noreferer">Another site</a>

Valid example

<a href="https://example.com/" target="_blank" rel="nofollow">Partner site</a>
<a href="https://example.org/" rel="noreferrer">Another site</a>

Learn more:

MDN Reference

MDN - rel attribute

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WHATWG Spec

HTML Living Standard - Links

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