I believe I just figured out a much nicer way for Tinderbox to generate HTML and XML.
The change isn't something you'll see: it's under the hood. The new code would be at once simpler, more expressive, and more standard-compliant. And it will be smaller and more testable, and so easier to support across platforms. (But it needs to be written and tested in the first place, so I don't think it's actually a way to shorten the TinderWin development schedule significantly.)
I've been working on the issue for ages, and then, there was the answer. That's how it works.