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Re: Shared Properties

From: Gregg Kellogg <gregg@kellogg-assoc.com>
Date: Sun, 2 Oct 2011 12:55:06 -0400
To: Jeni Tennison <jeni@jenitennison.com>
CC: "public-html-data-tf@w3.org" <public-html-data-tf@w3.org>
Message-ID: <C1A41A3C-7DDF-4948-B5FC-DF51DB72CB77@greggkellogg.net>
On Oct 2, 2011, at 4:20 AM, Jeni Tennison wrote:

> Pages that contain information about several entities often include common headers or footers that indicated a shared property for those entities. There is an example at the bottom of the section on Associating names with items in the microdata spec [1], where multiple images share a license. Toby Inkster raises the examples of Wikipedia disambiguation pages or tables with groupings [2].
> These use cases aren't particularly strong. Hixie has indicated [3] that the support for this in microdata, using itemref, was a side-effect of needing to support microdata fragments that are split across the page rather than due to a specific use case.
> Does anyone have any good real-world use cases from their own experience for shared properties? Would anyone volunteer to go looking for pages where the workaround (repeating properties within each entity) would be onerous?
> # Microformats #
> Microformats don't seem to have any support for shared properties; it looks as if users have to use hidden elements (empty spans) to add the shared properties within the wrapper element for each entity.

Microformats have the Include Pattern [7], which is similar to @itemref. Basically, this is a way to reference information defined in one part of the document from another:

	<a class="include" href="#author">Gregg Kellogg</a>
	<div class="vcard" id="author">
	  <div class="fn">Gregg Kellogg</div>

> # RDFa #
> RDFa doesn't have support for shared properties. The RDFWAWG ISSUE-105 [4] is about introducing such a mechanism, but Manu's draft response [5] indicates that it won't happen -- there are technical and backwards compatibility implications for RDFa processors -- unless there's a really good use case.

As you mention, there is little support for @itemref functionality in RDFa. [5] indicates possible support for multi-valued @about properties, which would support some of the same use cases, but without some pretty good use cases demonstrated, I don't know if it will get enough support to go through. However, it is a fairly simple change to the RDFa processing algorithm, would be reasonably easy for existing implementations to adopt, and poses no backwards compatibility issues. This might look like the following:

	<image src="foo.jpg/>
	<image src="bar.jpg/>
	<a about="foo.jpg bar.jpg" rel="cc:license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons B-NC-ND License</a>

> # Microdata #
> Microdata's itemref attribute [6] enables it to support multiple items with shared properties.
> If you have pages containing multiple entities which share properties described elsewhere in the page, use microdata.
> [1]: http://dev.w3.org/html5/md/#associating-names-with-items
> [2]: http://lists.w3.org/Archives/Public/public-rdfa-wg/2011Sep/0083.html
> [3]: https://plus.google.com/112095156983892490612/posts/NoCwrzYSHKP
> [4]: http://www.w3.org/2010/02/rdfa/track/issues/105
> [5]: http://lists.w3.org/Archives/Public/public-rdfa-wg/2011Sep/0066.html
> [6]: http://dev.w3.org/html5/md/#attr-itemref
> -- 
> Jeni Tennison
> http://www.jenitennison.com


[7] http://microformats.org/wiki/include-pattern
Received on Sunday, 2 October 2011 16:55:52 UTC

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