No time for details here, but a lot of progress made. In particular, I've forced the presence of id attributes in the output which begin with the primary xml:id of the document, and that means I can now write CSS rules based on any element ancestored by an element with an id beginning with the primary id, so I can write e.g. Stow 1598 rules into the primary_source.css stylesheet. That's better than encoding them in the document itself, as rendition elements, because we're rendering in ways that are not true to the original document.