---I don't really like this answer. The markup ought to be defined in a way that can represent authors' needs and cross-references to things in a document clearly are a need. I don't want to push things back into "the document product system" (if we start doing that, we might as well tell people to work in something that writes Acrobat and forget this markup language stuff).
Further, I don't think it's quite as simple on the production side as Lon suggests - you're going to have to parse the document, assuming you want to be able to support hierarchical numbering. It seems pretty ugly to me to require both the producer and the consumer side processes to parse the document. Server-side generation of the cross-references also means they would (presumably) not be modifiable by the style sheet mechanism, which seems like a pretty major failing.
On the other hand, it clearly is a problem that worst-case cross references couldn't be resolved until the document had been completely parsed. While many common uses of the facility would be local and non-pathological, it is pretty common to put an index at the front of a document and if the index included section numbers, that would be pretty pathological. Of course, the browser could render "unknown-cross-reference" glyphs for the forward references until it could resolve them; I don't think this would be excessively unpleasant (any more than it is a major jar now when the browser presents the first page, then decides it needs scroll bars and redraws it).
I am still inclined to think we should have a cross-referencing facility, with appropriate warnings to authors about the danger of long forward references.
scott
-- scott preece motorola/mcg urbana design center 1101 e. university, urbana, il 61801 phone: 217-384-8589 fax: 217-384-8550 internet mail: preece@urbana.mcd.mot.com