An anchor may also be specified by providing a value to the SRC
attribute on any HTML element which supports that attribute.
For example:
2b. Targets
A target is any addressable object or resource which typically
serves as the destination of a hypertext link. The destination
may be another HTML document, a fragment within the same or
another HTML document, or any other type of object or resource.
A target may also be an aggregate link which can be presented as
a list of possible targets from which a user can select. A Table
of Contents may be an example of an aggregate link. The result of
a query is an other example.
Any addressable object may serve as the target of a hypertext
link. Typically, a target is addressed by specifying a URL/URI as
the value of an HREF or SRC attribute on HTML elements which
support those attributes.
HTML also provides a number of language elements which may be
used to identify a target within an HTML document and to specify
Murray & Quin [Page 4]
INTERNET DRAFT Hypertext Links in HTML December 1995
the base location from which relative addresses should be
formed.
There are four ways to specify a target within an HTML document:
* the BASE element
* the NAME attribute of the A element
* the ID attribute of various elements, including A and LINK
(HTML 3.0 proposal)
* the NAME attribute of the LINK element
The base location of a document may be recorded in the BASE
element in the HEAD of the document. The base location is the
address from which all relative URL addresses are to be formed.
For example:
A target may also be specified as an attribute to a LINK element. A LINK element may only be used within the head of an HTML document, and it has no content. For example:
3. The LINK and A Elements and Their Attributes The LINK and A elements share a set of common attributes. Except where noted, the semantics of those attributes is the same. 3a. The LINK Element The LINK element indicates a hypertext link relationship between the document in which it is found and some other object. Any number of LINK elements may be used within the head of an HTML document. The LINK element is empty (does not have a closing tag). The LINK element takes the same attributes as the A (anchor) element. The hypertext link described by the LINK element is not typically represented within the text area of an HTML user agent. Instead, an HTML user agent is free to either ignore any LINK element and the hypertext link associated with it, or to represent the hypertext link in some other way. Presenting hypertext links as active icons in a toolbar is one way to present them to the user. Another may be to present the target document in a concurrent window, such as with a table of contents. Murray & Quin [Page 5] INTERNET DRAFT Hypertext Links in HTML December 1995 3b. The A Element The A element is used to indicate the start (anchor) or end (target) of a hypertext link within the body of an HTML document. The hypertext link described by the A element is typically represented as a highlighted object (word, phrase, graphical image, etc.) within the text area of an HTML user agent. 3c. Common Attributes For the purposes of this discussion, the following is a list with descriptions of the most important common attributes. All of the attributes listed here are part of HTML 2.0 except for CLASS, ID and MD, which are discussed in Dave Raggett's Internet Draft on HTML 3.0. CLASS The CLASS attribute value is used to subclass the hypertext link. The CLASS attribute is most often used as a generalized identifier to which style information may be attached by a stylesheet mechanism. The CLASS attribute may also be used to subclass LINK elements, thereby differentiating hypertext links with common REL or REV values. This may provide unambiguous syntax, for example, for multiple LINK elements with REL=NEXT within a document. Thus, alternate paths through a document can be coded within the document. For example: Multiple CLASS values may be specified. The potential list of CLASS attribute values is open-ended. However, practical application will likely require definition and specification of at least a small set of accepted keywords, and agreement on processing expectations for arbitrary keywords. The keywords used in the previous example are typical of the type of keywords which may be specified, but are not proposed. HREF The HREF attribute value specifies the location of a destination or resource, expressed in the Universal Resource Identifier (URI) notation. Only one HREF value may be specified. An HTML user agent may present the value of the HREF attribute in an information area when the user positions the mouse over the anchor or otherwise indicates interest in the anchor. ID The ID attribute specifies an SGML identifier used as the target of hypertext links or for naming particular elements in associated style sheets. Only one ID value may be specified. The attribute value must be unique within the document. Murray & Quin [Page 6] INTERNET DRAFT Hypertext Links in HTML December 1995 MD The MD attribute specifies a message digest or cryptographic checksum for the target of the hypertext link. This attribute is used by a user agent to verify that the linked object is the same one that the author intended. NAME The NAME attribute specifies a named location within an HTML document and is used in forming addresses to target specific locations within an HTML document. Only one NAME value may be specified. The attribute value must be unique within the document. REL The REL attribute specifies the relationship of the target to the anchor. For example, REL=NEXT is used to indicate that the target is the next logical document in an author- specified sequence. The REL attribute can also be used to support search for links serving particular relationships. Multiple REL values may be specified. Aggregate links can be formed by including multiple LINK elements with equivalent REL values. Activating the link, in that case, may lead to a virtual menu from which the user can make a selection. The nature of a link relationship is not always obvious from the attribute value. Section 4 (Meaning of REL and REV Attribute Values) describes commonly accepted values. Designers of HTML user agents can use these descriptions as a guide to implementation of browser or agent behavior. REV The REV attribute specifies the relationship of the anchor to the target. For example, REV=TOP is used to indicate that the anchor is the top of an author-specified hierarchical tree of which the target is a branch or node. Multiple REV values may be specified. A Table of Contents may contain a series of anchors which specify REV=TOC. The nature of a link relationship is not always obvious from the attribute value. Section 4 (Meaning of REL and REV Attribute Values) describes commonly accepted values. Designers of HTML user agents can use these descriptions as a guide to implementation of browser or agent behavior. TITLE The TITLE attribute is typically used to describe the linked object specified by the HREF attribute. The attribute value is a character string which may include spaces and punctuation. An HTML user agent may present the value of the TITLE attribute in an information area when the user positions the mouse over the anchor or otherwise indicates interest in the anchor. Authors/publishers can thereby provide greater context to the user to aid them in making decisions about whether to traverse a hypertext link. Murray & Quin [Page 7] INTERNET DRAFT Hypertext Links in HTML December 1995 An HTML user agent may also use the value of the TITLE attribute to set the value of an email subject field when the HREF value uses the `mailto:' scheme. In this way, the subject field may be preset with value that the author's mail agent can recognize and process. Although the HREF and TITLE attributes have been singled out in the list above, any of these attribute values may be presented in an information area when the user positions the mouse over the anchor or otherwise indicates interest in the anchor. For example, an HTML user agent which is also an HTML editor may display the value of ID or NAME attributes. 4. The REL and REV Attribute Values The potential list of valid values for REL and REV is open-ended, and this document is not intended to preclude the use or adoption of other or additional values. In fact, it is anticipated that hypertext applications which support specific knowledge domains will need to develop specialized sets of keywords. It is hoped that the development of extensions will not occur in isolation, and that coordination of extensions among various interested parties will prevent namespace contention or collision. Further, HTML user agents should continue to be liberal in accepting new or alternate values, inasmuch as any name token is a legal value. The HTML specification declares that REL/REV values are SGML name tokens. That is, within the previously described syntactical constraints, a REL or REV relationship value may be any arbitrary, author-defined value that the author or publisher considers important. In some cases, the HTML user agent may choose to present relationships that it recognizes in a richer style, while continuing to present unfamiliar ones in the default style. The REL and REV attributes are defined as NAMES in the SGML DTD for the HyperText Markup Language (HTML). As such, the legal values which may be assigned to the REL or REV attribute are zero or more name tokens. Name tokens are case-insensitive, must begin with an alpha character, may include digits (0-9), period or hyphen, and may be separated by spaces. The name tokens listed and described in this section are being recommended as commonly accepted relationships between and among objects in a hypertext collection. When a single name token is specified as a REL or REV attribute, double quotes surrounding the attribute value are optional. When multiple name tokens are specified, double quotes are mandatory. previous previous next 4a. Legacy The following are REL values which were known to be used as values of the REL and REV attributes on the World Wide Web in December 1995. MADE The REV=MADE relationship has been used to identify the author or "maker" of an HTML document. Typical HREF values include a Murray & Quin [Page 8] INTERNET DRAFT Hypertext Links in HTML December 1995 `mailto:' URI or the URL of the author's home page. Example: Author NEXT/PREVIOUS/TOC/INDEX/NAVIGATOR These values are described below, are used by SCO in its online documentation and context- sensitive help system. 4b. Browser-defined Links Some keywords are reserved and should not be used as REL/REV values. HTML user agents typically provide a mechanism for navigating through the recent history of a user's access to documents; traditionally these operations are referred to as "back" and "forward". These mechanisms allow a user to step back through the documents which led to the current location and then forward again to retrace the path. Additionally, most user agents provide a mechanism to immediately return to a user-defined location, traditionally referred to as the home page, or "home". Since these browser actions are internally implemented by the browser, REL/REV keywords associated with these relationships are disallowed. HOME RESERVED. Defined by the user (for example, using an environment variable or preference, e.g. WWW_HOME). This relationship may not be overridden; HTML user agents should ignore any author-supplied REL=HOME setting. BACK RESERVED. Defined by the browser. This relationship may not be overridden; HTML user agents should ignore any author-supplied REL=BACK setting. FORWARD RESERVED. Defined by the browser. This relationship may not be overridden; HTML user agents should ignore any author-supplied REL=FORWARD setting. 4c. Navigational Node Links Navigational nodes are commonly used document objects which are designed by authors to assist the user in navigating through a closed or extended document set. The most familiar and common form of navigational node is a table of contents, which is a well known publishing device used for enumerating and ordering the contents of a closed document set. CONTENTS or TOC The TOC relationship identifies a Table of Contents. When REL=TOC, the target document is the Table of Contents for the current document, or for the collection of documents of which the current document is a member. When REV=TOC, the current document is a Table of Contents and the target document is a related document. Murray & Quin [Page 9] INTERNET DRAFT Hypertext Links in HTML December 1995 When REL=TOC and REV=TOC it indicates that the current document is a Table of Contents and the target document is also a Table of Contents. Additional REL/REV values may be used to specify the relationship between the two, such as PARENT/CHILD. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. Or, if capable, an HTML user agent may present the Table of Contents in a concurrent window or pane, highlighting the current document. INDEX The INDEX relationship identifies an index. When REL=INDEX, the target document is an index for the current document, or for the collection of documents of which the current document is a member. When REV=INDEX, the current document is an index. Additional REL/REV values may be used to further specify the relationship between the two ends of the link. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. An index may be presented as an HTML document which is organized and presented in a style reminiscent of a paper-based index. An index may also be presented as a form-based query into a full- text search database. NAVIGATOR The NAVIGATOR relationship identifies a navigational aid. When REL=NAVIGATOR, the target document is a navigational aid. A navigational aid may consist of a whole or partial Table of Contents, a list of related documents, an indication of the current document's location within a document hierarchy, or any other information which may be useful to the user. When REV=NAVIGATOR, the current document is a navigational aid. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. 4d. Hierarchy Links It is quite common for documents to be developed or defined using a hierarchical model, or tree-like structure. The keywords listed below may be used within HTML documents to identify the hierarchical relationship of closely related nodes, such as the immediate parent, siblings and children. In addition, the TOP keyword may be used to identify the logical top (or root, depending on your perspective) of a hierarchical or tree-like structure. The entire set of relationships may be used by a user agent to build a map of the hierarchical structure(s) of which the current document is a node. Hypertext links to documents identified with PARENT and TOP values are more likely to be Murray & Quin [Page 10] INTERNET DRAFT Hypertext Links in HTML December 1995 accessible through an icon or other mechanism than documents identified with CHILD or SIBLING. CHILD The CHILD relationship identifies a subordinate or subdocument. Any document may have multiple CHILD documents within the same hierarchy. When REL=CHILD, the target document is a hierarchical child, or subdocument, of the current document. When REV=CHILD, the current document is the hierarchical child, or subdocument, of the target. PARENT The PARENT relationship identifies the superior or container node. When REL=PARENT, the target document is the hierarchical parent, or container, of the current document. When REV=PARENT, the current document is the hierarchical parent, or container, of the target. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. SIBLING The SIBLING relationship identifies a sibling in the current hierarchy. Any document may have multiple SIBLING documents within the same hierarchy. When REL=SIBLING, the target document is a child of a common parent, or a hierarchical peer of the current document. REL and REV have equivalent meanings for the SIBLING relationship. TOP or ORIGIN The TOP relationship identifies the logical top of a hierarchical tree of which the current document is a branch. BEGIN is a functional equivalent to TOP, if only one of these values is specified. When REL=TOP, the target document is the logical top node of the tree. When REV=TOP, the current document is the logical top of the tree. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. NOTE: ORIGIN has been suggested as an alternative to TOP to provide metaphorical consistency with PARENT/CHILD/SIBLING. Comments are encouraged. 4e. Sequence Links Given a set of documents, it is possible and often desirable to specify linear sequences to navigate through the set. A book, for example, is often organized as a linear sequence. With sequence Murray & Quin [Page 11] INTERNET DRAFT Hypertext Links in HTML December 1995 links in each document, a user agent can step through or gather an entire book programmatically. BEGIN or FIRST The BEGIN relationship identifies the author- defined start of a sequence of documents of which the current document is a node. TOP is a functional equivalent to BEGIN when only one of these values is specified. When REL=BEGIN, the target document is the beginning of the sequence. When REV=BEGIN, the current document is the beginning of the sequence. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. END or LAST The END relationship identifies the author defined end of a sequence of documents of which the current document is a node. TOP is a functional equivalent to END when only one is specified. When REL=END, the target document is the end of the sequence. When REV=END, the current document is the end of the sequence. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. NEXT The NEXT relationship identifies the next document in an author-defined sequence of documents, such as a linear book. When REL=NEXT, the target document is next after the current document. When REV=NEXT, the current document is next after the target. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. PREVIOUS or PREV The PREVIOUS relationship identifies the previous document in an author-defined sequence of documents, such as a linear book. When REL=PREVIOUS, the target document is previous to the current document. When REV=PREVIOUS, the current document is previous to the target. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. 4f. Related Documents BIBLIOENTRY The BIBLIOENTRY relationship identifies a bibliographic entry. BIBLIOENTRY would most typically be specified on an A element, as it would specify a hypertext link between a citation and a Murray & Quin [Page 12] INTERNET DRAFT Hypertext Links in HTML December 1995 bibliographic entry describing the citation. Example: As We May Think The resource identified by this link may take any form desired by the author/publisher. A bibliographic entry may be presented in the style of a paper-based bibliographic entry, or it may be presented as the result of a database query. BIBLIOGRAPHY The BIBLIOGRAPHY relationship identifies a bibliography. The resource identified by this link may take any form desired by the author/publisher. A bibliography may be presented as an HTML document which is organized and presented in a style reminiscent of a paper-based bibliography. A bibliography may also be presented as a form-based query into a bibliographic database. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present a labeled icon in a tool bar. CITATION The CITATION relationship identifies a bibliographic citation. When REL=CITATION, the target is a bibliographic citation. The anchor, in this case, may be a bibliographic entry. The anchor may also be a reference, thus allowing the reader a way to locate the citation: ... as described by Tim Berners-Lee [1] ... When REV=CITATION, the anchor is a citation. Typically, the anchor would also be enclosed within a CITE element as shown in the example below. The example shown here also corresponds to the previous example, serving as its target by use of the NAME attribute. ... is described in Tim Berners-Lee's The HyperText Markup Language ... NOTE: an alternative (and preferred) approach would be to add a URI-valued attribute (HREF?) to the HTML CITE element. DEFINITION The DEFINITION relationship identifies a definition of a term. Definitions may be, but are not necessarily, contained within a glossary. DEFINITION would most typically be specified on an A element, as it would specify a hypertext link from a term to its definition. HTTP FOOTNOTE The FOOTNOTE relationship identifies a footnote. Murray & Quin [Page 13] INTERNET DRAFT Hypertext Links in HTML December 1995 When REL=FOOTNOTE is specified on an A element, the anchor is a footnote marker and the target is a footnote. This can be used to link from the footnote marker (or a highlighted word, phrase, etc.) to an HTML document which contains the footnote text, or to a portion of the same document (see REV=FOOTNOTE). When REL=FOOTNOTE is specified on a LINK element, it can specify a hypertext link to a set of footnotes which are related to the current document, or to a set of end-notes. When REV=FOOTNOTE is specified on an A element, the anchor is a footnote; that is, the actual content of the footnote, as opposed to a footnote marker. In this case, the target specified by the HREF value, if any, is the footnote marker. It has been suggested that the combination of REV=FOOTNOTE and NAME=... on an A element may be used to imply that the enclosed content not be rendered until a link to it is explicitly traversed, at which time it can be presented in a popup window. This would allow for the inclusion of footnote text within a document that would not be visible until the reader wanted it to be presented. Developers of user agents are free to experiment with this proposed feature, but there is no requirement that it be implemented. GLOSSARY The GLOSSARY relationship identifies a glossary. When REL=GLOSSARY, the target document is a glossary. When REV=GLOSSARY, the current document is a glossary. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. A glossary may be directly presented as an HTML document which is organized and presented in a style reminiscent of a paper-based glossary. A glossary may also be accessed through an intermediary query mechanism. For example, the user highlights a word or phrase and presses the glossary button, thereby accessing the linked object and passing the highlighted text as an argument. The server returns the glossary entry relevant to the highlighted word. 4g. Meta Documents There are classes of information which are not intrinsic to a document, but for which a clear and unambiguous association is often useful or even necessary. This section defines a small set of keywords which are related to ownership and legal notices. Any attempt to rigorously define a closed set of meta- data classes, types, and formats is doomed to failure, partly due to the need for ongoing experimentation. Hence, the META keyword may be used to identify meta documents which do not necesarily have a clear or unambiguous definition. The content of the target node may be as specific format as a MARC record or an FGDC record, or it may be an author-defined format. Murray & Quin [Page 14] INTERNET DRAFT Hypertext Links in HTML December 1995 For each of the relationship keywords listed in this section, if the relationship is specified with REL in a LINK element, an HTML user agent may present a labeled icon in a tool bar. AUTHOR The AUTHOR relationship identifies a hypertext link to an author. The hypertext link may be to the author's home page, a biography, an audio or video clip, or an agent which sends mail to the author (e.g., using the `mailto:' scheme). COPYRIGHT The COPYRIGHT relationship identifies a hypertext link to a copyright notice. While it is arguable whether a copyright notice is required in every HTML file to assert copyright protection on it, there is clearly a desire to express copyright notice among a sufficient portion of the user community to justify support. A basic copyright notice for this document may simply state: "Copyright 1995 by Murray C. Maloney". It may be desirable, in place of or in addition to such a notice, to have a hypertext link between each HTML document in a set and a single copyright notice, as in the following examples: Copyright 1995 by Murray C. Maloney DISCLAIMER The DISCLAIMER relationship identifies a hypertext link to a legal disclaimer. Usage is expected to be similar to that of the COPYRIGHT hypertext link. As with the copyright notice, there is no intention or expectation that such a link would be the only way to express a disclaimer. EDITOR The EDITOR relationship identifies a hypertext link to an editor. Usage is expected to be similar to that of the AUTHOR hypertext link. META The META relationship identifies a hypertext link to a node which contains meta-information related to the current document. This is intended to be a generalized meta-data relationship descriptor. PUBLISHER The PUBLISHER relationship identifies a hypertext link to a publisher. Usage is expected to be similar to that of the AUTHOR hypertext link. TRADEMARK The TRADEMARK relationship identifies a hypertext link to a trademark notice. Usage is expected to be similar to that of the COPYRIGHT hypertext link. Murray & Quin [Page 15] INTERNET DRAFT Hypertext Links in HTML December 1995 4f. Other REL and REV Values Under Discussion The POINTER keyword is an invention of the author. The BANNER, BOOKMARK, HOTLIST and STYLESHEET keywords are described in Dave Raggett's Internet Draft on HTML 3.0. Recent discussions tend to indicate that these keywords may not be appropriate for use as REL/REV values. Dave Raggett's further explanation and justification is needed before any further discussion or decision can be made as to the future status of these keywords. The LANG attribute is described in Dave Raggett's Internet Draft on HTML 3.0. It has been applied to various HTML elements, not including the LINK and A elements. The author suggests that LANG is a useful attribute to apply to the LINK and A elements. See also the discussion of REL=TRANSLATION. BANNER The BANNER relationship identifies a document banner. When REL=BANNER, the target document is to be included within the current document as a banner. A banner is typically used for corporate logos, custom toolbars, and other information which would not typically be scrolled with the body of a document. When REV=BANNER, the current document is a banner. This may be used, in future, to provide error-checking or to prevent the use of a document as a banner unless it has been explicitly identified as a valid source. (Or not! Sorry, I was reaching for a useful meaning.) Compelling arguments have been made against the need for a REL=BANNER value, which is simply a special case of the INCLUDE mechanism. BOOKMARK The BOOKMARK relationship identifies a bookmark. Bookmarks are used to provide direct links to key entry points into an extended document. The TITLE attribute may be used to label the bookmark. Several bookmarks may be defined in each document, and provide a means for orienting users in extended documents. HOTLIST RESERVED: This keyword has been proposed by Dave Raggett. Its meaning and purpose require further explanation. A placeholder is being maintained until such time as Dave has had an opportunity to provide further explanation, examples, discussion and justification. If the hypertext link is specified with REL in a LINK element, an HTML user agent may present an icon in a tool bar. LANG The LANG attribute indicates the language of the target document. Murray & Quin [Page 16] INTERNET DRAFT Hypertext Links in HTML December 1995 The LANG attribute is optional and has no default value. It may be used for purely informational purposes by an HTML user agent, or by a robot for language classification. Used in combinatiuon with a proposed REL=TRANSLATION and a user's language preference setting, an HTML user agent may intelligently select from a collection of otherwise equivalent hypertext links expressed with the LINK element. If the user's language preference is not available, the user agent may present a virtual menu of language options. See the Internet Draft on the Internatiolisation of HTML for a definition of the values of this attribute. POINTER The pointer relationship identifies a hypertext pointer. That is, this is a way to do indirection in HTML. When REV=POINTER, the anchor is a pointer to the target document. When a hypertext link is traversed to a LINK or A element with REV=POINTER, the target specified by the HREF value should be traversed, and so on, until a target without REV=POINTER is retrieved. When REL=POINTER, the target is a pointer to the real target. This value can be used by a user agent to perform a pre-fetch of the specified target for evaluation until the real target is reached. NOTE: The authors propose that the NAME attribute be removed from the LINK element, or that a practical use for it should be defined. For example, hypertext indirection can be specified by providing both a NAME and an HREF value on the LINK element, in combination with a specific REL or REV value, such as POINTER. Some support exists among members of the HTML Working Group to provide for hypertext indirection with the LINK element. There is no other reason for an author to define a target by using the NAME attribute on a LINK element, since the resulting target address is functionally equivalent to the address of the document in which such a target is defined. STYLESHEET The STYLESHEET relationship identifies a stylesheet. When REL=STYLESHEET, the target document is a stylesheet. When associated with a LINK element, the author/publisher is expressing an expectation that the target stylesheet will be applied by the HTML user agent. When associated with an A element, an HTML user agent may simply retrieve the target stylesheet for display, or it may launch a stylesheet editor with the target stylesheet. When REV=STYLESHEET, the current document is a stylesheet and the target document may be a demonstration of its use. In general, it is not anticipated that stylesheets will contain LINK or A elements, as they are not projected to be HTML Murray & Quin [Page 17] INTERNET DRAFT Hypertext Links in HTML December 1995 documents. TRANSLATION The TRANSLATION relationship specifies a translation to another language. When REL=TRANSLATION, the target is a translation to another language. This value will most typically be used with the LINK element, in combination with specification of the target document's language as a LANG attribute value. Presumably, REL=TRANSLATION can be used with the A element to specify a translation of a document fragment, such as a phrase in a foreign language. When REV=TRANSLATION, the current document, or document fragment, is a translation of the target. URC The URC relationship identifies a Uniform Resource Catalogue for the current document. This keyword has been proposed by Dave Raggett. Its meaning and purpose have not been explained to the author, but a placeholder is being maintained until such time as Dave has had an opportunity provide explanation, examples, discussion and justification. 5. Hypertext Includes There have been many discussions in various forums which clearly indicate that hypertext includes are a desired feature of the HTML language, and for which widespread user agent support is needed. There are, apparently, three popular syntactic approaches to inclusion: specifying INCLUDE as REL value on the A and LINK elements, specifying a newly-defined and specially- purposed INCLUDE element (which would presumably also have REL and REV attributes), and using SGML entities. Each is described and discussed below, but no arguments are presented. In either case, there is an expectation that processing of an INCLUDE link would result in the INCLUDE value being deleted from the REL list and inserted into the REV list. In other words, a REL=INCLUDE indicates that the inclusion has yet to happen, while a REV=INCLUDE indicates that the inclusion has already happened. It is helpful, for legal and copyright purposes, that included material be identifiable at all times. Note that the form of inclusion referred to here is also known as Transclusion, or as client side inclusion. It may happen either automatically, when a document is loaded into an HTML client, or explicitly: for example when the user selects an icon, included text may appear at that point in the current document. 5a. INCLUDE as a REL or REV Value INCLUDE relationship identifies a document for inclusion. When REL=INCLUDE, the target document should be included at the anchor location. This value is typically, though not always, used Murray & Quin [Page 18] INTERNET DRAFT Hypertext Links in HTML December 1995 with the A element. Using this value on a LINK element implies that the included document only contains information which is valid within the HEAD of an HTML document. When REV=INCLUDE, the current document is identifying itself as an included document in the target document. 5b. INCLUDE as an Element An anchor may also be specified using a newly-defined INCLUDE element: