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Andrew Kirkpatrick in http://lists.w3.org/Archives/Public/public-html-a11y/2013Aug/0069.html suggests the spec require that longdescs which represent part of a document, e.g. in a document containing several different descriptions, be well-formed fragments, and that user agents be required to use these.
*** Bug 23292 has been marked as a duplicate of this bug. ***
Andrew, Thanks for taking the time to consider longdesc in the context of specific WCAG techniques, specifically the behavior of user agents when a longdesc refers to a document fragment. The Task Force has intentionally refrained from requiring any specific behavior in this respect from user agents. There is a requirement in the document that authors SHOULD put descriptions in well-formed fragments (which may include an entire page). Because there is existing, and will likely continue to be new content that does not meet this requirement, we do not propose a corresponding requirement on user agents.