Canonical reference for rel=“foo”

Question

The answers to a lot of google-juice questions seem to involve the use of rel= in a tags (eg rel="canonical"). Is there an exhaustive list somewhere of all of the possible options for this tag and what they mean? I found this from W3 schools, but it doesn't have canonical on it and I'm wondering what else it's missing.

Answer 1

The official link types as defined by the W3C specify the following for HTML 4.01:

http://www.w3.org/TR/html401/types.html#type-links

In HTML 5 the W3C appear to have handed off registration and management of these link types to the microformats folks. See section 4.12.4.14` Other link types:

Extensions to the predefined set of link types may be registered in the microformats wiki existing-rel-values page. [MFREL]

Anyone is free to edit the microformats wiki existing-rel-values page at any time to add a type.

There is also a rel registry which can be found here:

http://microformats.org/wiki/rel-registry

If you scroll down you can see the stable, draft, provisional etc rel types. It's fairly comprehensive.

IANA also define a list of link relations which can be found here:

http://www.iana.org/assignments/link-relations/link-relations.xml

This links to RFC5988:

http://tools.ietf.org/html/rfc5988

Answer 2

As far as SEO goes there are not a lot of values officially recognized for the rel attribute when used in the <a> tag. As far as I know they are:

• nofollow
• Google authorship markup

There are other values you can add the rel attribute that do have semantic meaning but they have not been given specific meaning by the search engines. That doesn't mean that they aren't useful in some other way, however. external is a common value for rel in an <a> tag to indicate an external link but isn't an official rel value nor does it have SEO value.

FYI, you should also check out schema.org for microformats. This is another way to add structure to your markup to help search engines recognize your data as being of a certain type and index accordingly.