The 1.1 lang spec for <fn/> says that the presence of the ID attribute on <fn/> affects how the footnote is rendered. Is that true?
If so, I think we need to mention it in the ID attribute section of the archspec. I'm imagining a tool vendor providing an option to "automatically generate IDs", not knowing that the presence of the ID attribute has processing implications.
Are there other elements for which the presence of @id has any effect, whatsoever?