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In fact, we might have more flexibility in the styling but it would probably be quite the same for typestting stuff (fop aims to be quite similar to tex for that) so it won't probably behave much different (except for the styling).
I don't know if FOP can handle css directly or if it does much of the styling itself taking only the document content and structure as an input. If so, there would only be the benefit of the tbook language.
Anyway it seems, tbook and fop are quite immature compared to LaTex right now.