Typed link

A typed link in a hypertext system is a link to another document or part of a document that includes information about the purpose of the link. For example, rather than merely pointing to the existence of a document, a link might also specify that the document supports the conclusion of the article pointing to it, that it contradicts the article pointing to it, that it is an older version of the document, that it serves to define the word next to the link, that it is an index to other documents of the same type, or some other relationship. This allows a user to take actions such as searching only certain types of links or displaying them differently. It may also allow browsing software to do things like pre-fetching documents it expects the user to browse.

Present HTML supports typed links from whole documents to whole documents with the <link> tag. For example, the tag <link rel="Contents" class=encyclopedia href="top"> specifies that the document "top" is a table of contents for the work that includes the document you are currently reading, and the tag <link rel="Next" class=encyclopedia href="chap3"> specifies that "chap3" is the next document in logical sequence after the one you are reading. There is also the reverse attribute.

The HTML standard states that both rel and rev can be added to the typical <a> tag. A Draft RFC to define standard link types for HTML was proposed in 1994, but expired without much comment.

Typed links, while not really a part of Internet/HTML based hypertext systems were a common feature in pre-Internet hypertext systems such as Xanadu, NoteCards, HyperWriter, IBIS/gIBIS and others. While typed links can be very useful, the lack of a standardized set of link attributes such as "Supports Position"/"Refutes Position" as well as the difficulty of applying the attributes has always hindered the use typed links beyond prototyping purposes. Any system of attributes would turn a hypertext corpus into a semantic web, and permit more sophisticated processing.