>         <link rel="chapter" href="#basics" title="Basics">
>             <link rel="section" href="#http" title="HTTP" />
>         </link>

This is essentially what IE's channel definition files do (as external
XML files).  They seem to have failed.  Whilst part of the failure may
be that people associated them with the business model, rather than 
their use to outline the structure, and those most likely to use the
structure may have been least happy with the business model and the
proprietory nature, it seems to me that the failure does indicate a
lack of interest in such a feature.

Another such indicator is the number of PDF documents that have an
empty structure tree.