rel-enclosure
From Microformats
rel="enclosure"
relEnclosure is a simple, open, format for indicating files to cache which is embeddable in (X)HTML, Atom, RSS, and arbitrary XML. relEnclosure is an elemental microformat, one of several microformat open standards.
| Table of contents |
Draft Specification 2005-06-21
Author
Kevin Marks (http://epeus.blogspot.com), Technorati, Inc. (http://technorati.com)
Copyright
This specification is (C) 2004-2008 by the authors. However, the authors intend to submit this specification to a standards body with a liberal copyright/licensing policy such as the GMPG (http://gmpg.org/), IETF (http://ietf.org/), and/or W3C (http://w3.org). Anyone wishing to contribute should read their copyright principles, policies and licenses (e.g. the GMPG Principles (http://gmpg.org/principles)) and agree to them, including licensing of all contributions under all required licenses (e.g. CC-by 1.0 (http://creativecommons.org/licenses/by/1.0/) and later), before contributing.
Patents
This specification is subject to a royalty free patent policy, e.g. per the W3C Patent Policy (http://www.w3.org/Consortium/Patent-Policy-20040205/), and IETF RFC3667 (http://www.ietf.org/rfc/rfc3667.txt) & RFC3668 (http://www.ietf.org/rfc/rfc3668.txt).
Abstract
relEnclosure is one of several microformats. By adding rel="enclosure" to a hyperlink, a page indicates that the destination of that hyperlink is intended to be downloaded and cached. E.g. with the following hyperlink:
<a href="http://homepage.mac.com/kevinmarks/wwdc2005.mov" rel="enclosure">WWDC 2005 Keynote with chapters</a>
the author indicates that the movie should be downloaded.
Multiple Enclosures
Multiple such rel="enclosure" hyperlinks indicate that several media items should be downloaded.
Question: What if there are several alternative formats, such as high- and low-bandwidth recordings? Since <link rel="alternate" href="..."> is already defined in XHTML for that purpose, so how should it interact with this new rel tag?
Response: rel="alternate" is defined in the HTML spec:
Designates substitute versions for the document in which the link occurs.
In the case of enclosure links "alternate" is not suited, because the media file is not a substitute version of the document in which the link occurs - it is a substitute version of another link on the document in which the link occurs.
Proposal: Multiple instances of the same enclosure (i.e. same video with different formats, size/resolution, bitrates, etc) could be grouped in a class="enclosure" block with multiple rel="enclosure" links. This would be analogous to <media:group> in MRSS (http://search.yahoo.com/mrss)
reply
- "In the case of enclosure links "alternate" is not suited, because the media file is not a substitute version of the document in which the link occurs "
- That's not necessarily the case; the linked file may be an audio reading of the text on the page, for instance. Andy Mabbett 02:37, 5 Oct 2007 (PDT)
XMDP profile
<dl class="profile">
<dt id="rel">rel</dt>
<dd><p>
<a rel="help" href="http://www.w3.org/TR/html401/struct/links.html#adef-rel">
HTML4 definition of the 'rel' attribute.</a>
Here is an additional value.</p>
<dl>
<dt id="enclosure"> enclosure </dt>
<dd>Indicates that the referred resource is intended to be downloaded and cached</dd>
</dl>
</dd>
</dl>
Implementations
This section is informative.
The following implementations have been developed which either generate or parse rel-enclosure links. If you have a rel-enclosure implementation, feel free to add it to the top of this list. Once the list grows too big, we'll make a separate wiki page.
- FeedBurner (http://www.feedburner.com): the SmartCast (http://forums.feedburner.com/viewtopic.php?t=20) service looks for the rel="enclosure" microformat and converts it to an enclosure in the feed.
- ODEO (http://odeo.com) publishes (http://odeo.com/blog/2005/07/adding-microformats-to-odeo.html) rel-enclosure links for user podcast MP3s.
- Video Pop-Up Link Maker (http://joshkinberg.com/popupmaker/): generates code for rel-enclosure links for videobloggers. Also generates Javascript to launch a pop-up window with the video embedded. (Also available at embedthevideo.com)
References
Normative References
- XMDP (http://gmpg.org/xmdp/)
Informative References
- The Atom Syndication Format (http://www.ietf.org/rfc/rfc4287.txt):
- 4.2.7.2 The "rel" Attribute
4. The value "enclosure" signifies that the IRI in the value of the
href attribute identifies a related resource which is potentially
large in size and might require special handling. For atom:link
elements with rel="enclosure", the length attribute SHOULD be
provided.
