In Principio (of the Web)...

From HTML - IETF draft 1.2 of 1993

element link - this document links to that one

Attribute rel="X"
The link to that document is of type X
E.g. <a href="gloss" rel="glossary">xxx</a.>
xxx is linked to something that is a glossary.

rel, rev are literal text values, for documents or hypertext