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Anne Van Kesteren pointed out in twitter that the idea of reflecting a content attribute is defined in HTML, and doesn't happen by default. I think the fix is to add a line saying the attribute reflects the value of the longdesc attribute, with a link to what reflecting is in the HTML spec. Probably a good idea to add a pointer in a glossary section too.
Hopefully fixed in https://dvcs.w3.org/hg/html-proposals/raw-file/753af00ced01/longdesc1/longdesc.html
fixed in editor's drafts, agreed in TF meeting