Typed links are an important feature of the web architecture. They are one of the original design issues.
The notion has been revivied in a recent draft in the IETF HTML working group.
To participate in the discussion about typed links, see: HTML discussion forums.
What follows is from a draft of the HTML spec circa 1991:Connolly, May 1996
Status: This list is not part of the standard. It is intended to illustrate the use of link relationships and to provide a framework for further development.
Additions to this list will be controlled by the HTML registration authority . Experimental values may be used on the condition that they begin with "X-".
Link relationship valies are NOT case sensitive. That is, "Made" and "made" have th esame meaning.
These values of the REL attribute of hypertext links have a significance defined here, and may be treated in special ways by HTML applications.
These relationships relate whole documents (objects), rather than particular anchors within them. If the relationship value is used with a link between anchors rather than whole documents, the semantics are considered to apply to the documents.
In the explanations which follows, A is the source document of the link and B is the destination document specified by the HREF attribute.
A relationship marked "Acyclic" has the property that no sequence of links with that relationship may be followed from any document back to itself. These types of links may therefore be used to define trees.
Most relationships (except where noted) are between the objecs themselves rather than the subjects of the objects. Objects may be documents, images, people (with mailto: URIs for example.)
B is a related index for a search by a user reading this document who asks for an index search function.
A document may have any number of index links, causing several indexes top be searched in a client-defined manner.
B must support SEARCH operations under its access protocol.
B is an index which should be used to resolve glossary queries in the document. (Typically, a double-click on a word which is not within an anchor).
A document may have any number of glossary links.
The information in B is additional to and subsidiary to that in A.
Annotation is used by one person to write the equivalent of "margin notes" or other criticism on another's document, for example.
Example: The relationship between a newsgroup and its articles.
Similar to Annotation, but there is no suggestion that B is subsidiary to A: A and B are on equal footings.
Example: The relationship between a mail message and its reply, a news article and its reply.
If this link is followed, the node at the end of it is embedded into the display of the source document.
In an ordered structure defined by the author, A precedes B, B is followed by A.
Any document may only have one link of this relationship, and/or one link of the reverse relationship.
Note: May be used to control navigational aids, generate printed material, etc. In conjunction with " subdocument ", may be used to define a tree such as a printed book made of hypertext document. The document can only have one such tree.
B is a lower part in the author's hierarchy to A. Acyclic. See also Precedes .
Whenever A is presented, B must also be presented. This implies that whenever A is retrieved, B must also be retrieved.
When the link is followed, the node B should be searched rather than presented. That is, where the client software allows it, the user should immediately be presented with a search panel and prompted for text. The search is then performed without an intermediate retrieval or presentation of the node B
B is a previous version of A.
B is a list of versions of A
A link reverse link must exist from B to A and to all other known versions of A.
Person (etc) described by node A is author of B
This information can be used for protection, and informing authors of interest, for sending mail to authors, etc.
The owner of an object carried resposabiliy for and authority over the object.
This information may be used for finding people responsible for incorrect information, etc. The creator (Made) and owner (Owns) of an object may be different.
Approval of objects is a method of attributing value, or fiability, to objects. One determination of a value of an object is a function of the set of objects which approve it.
A reviewed journal, for example, may operate by approving articles. This could be expressed by an approval link from the journal itself to the article. In the view of the web as an encyclopaedia, approval links one to filter information which has a certain quality according to some standard.
A and B are objects representing assertions. The assertion A supports the assertion B. This may be used to overlay a weak semantics of argument onto the web. For example, giving such relaionships within discussions in this way will allow arguments to be analysed by machine and followed by people with greater ease. See also: refutes.
This is the opposite of "Supports", indicating that A is a proposition which refutes proposition B.
A includes B, B is part of A. For example, a person described by document A is a part of the group described by document B.
Note: This relationship conveys semantics about objects described by objects, rather than the documents themselves.
Person (etc) described by A is interested in node B.
This information can be used for notification of changes.
Typically, this is a request that, when object B changes in some way, a new link is made to object A.
The phrase "object B changes" may be interpreted narrowly (as "B itself changes") or widely (as "B or anythink linked to it or related to it closely changes"). The amount of change considered worth notifying people about is also subject to interpretation, varying from bit changes in the source to a "new edition" statement by the publisher.