The comments in the DTD that the document references describe an interesting evolution of its linking architecture: there was an attempt, later abandoned, to keep it in line with XLink; I was tickled to see the phrase "architectural form" come up in one comment. Ultimately, they modeled the links around the relationships between their particular document types instead of trying to shoehorn these relationships into some wider linking standard, and then the XSLT stylesheet that prepares it for web delivery turns the links into a/@href links.