>>> What I'm trying to get at is the fundamental rational between what appears to be two extremes not necessarily compatible, and why we ended up with only the later.
Actually, and for a very long time before it became "cool" ;) to talk about this, it's really only been the latter, except that the "markup" for the rest of the document is implicit. Adding a "root" tag for the whole document simply formalises and makes machine-readable (although you could have done this a variety of other ways, like filename extensions) what "documentalists" of all kinds have been doing for a very long time; (more or less formally) identifying integral units of documentation.
A MARC21 serialisation has message headers to separate different catalogue records in the stream - those are short documents, generally, although they can potentially get very long. And every element, as well as the whole thing, is marked up.
