RELs

Rels, short for relation types, are a way to describe the relationship between the current document and a linked resource. The term “rels” is used to refer to the set of possible relation tokens that can be declared with the rel attribute in HTML and related linking contexts. The rel attribute is most commonly applied to anchor, area, and link elements.

In HTML, rel values are specified as a space-separated list of tokens. A token can be a registered relation type, such as stylesheet, alternate, author, canonical, next, prev, license, or license, as well as URIs that identify a specific relation type. Registered tokens are maintained in standards documents and in the IANA Link Relations registry. Some rel values are recommended by standards to guide user agents and crawlers in handling the linked resource.

Common uses include linking CSS stylesheets with rel="stylesheet", pointing to the canonical URL of a page with rel="canonical", or indicating alternate representations (for example rel="alternate" type="application/rss+xml"). Rel values also appear in navigational contexts (rel="next" and rel="prev") and in metadata contexts (rel="author", rel="license"). In addition, rel can convey security and privacy hints when used with target="_blank", such as rel="noopener" or rel="noreferrer", to mitigate reverse tabnabbing.

Notes and evolution: the rel attribute concept is formalized in RFC 5988/8288 (Web Linking) and is governed by the IANA Link Relations registry. The HTML rev attribute, a predecessor in HTML 4, is now deprecated in favor of rel. Some values, like “nofollow,” “sponsored,” and “ugc,” have been adopted by search engines to influence crawling and ranking behavior.