> I request that the XHTML2 specification include an algorithm that exactly 
> defines how to obtain a document outline from a given XHTML2 document, so 
> that user agents can interoperabily represent document outlines.

This is obviously taking the "there can be no error cases" line, but
it also has shades of the separator argument.  Although Hn isn't empty,
it is still structurally a separator.