>         <link rel="chapter" href="#introduction" title="Introduction" />
>         <link rel="chapter" href="#basics" title="Basics">

I missed that these were all local fragments.  For a properly written
HTML document, this nesting structure will already be available and 
some browsers, as well the html2ps program, used to create the PDF
versions of the HTML 4.x specifications, already convert this into
a collapsible tree.

The only added value seem to be the rel= attribute, but this seems to
subclassing.  In fact, if you want to mark the section rather than the
headings, you can sub-class DIVs in HTML 4 and sections in XHTML 2.
Otherwise, just sub-class the Hn elements.