Ultimately, it is not a life or death decision, as conversion between
DTDs is not a vast undertaking.
XML version of HTML? a clean break and richer
vocabulary would be good.
Docbook? well-understood, with rich "computerese"
markup, but verbose and underdocumented
Use namespaces, and pull in vocabulary from all over (tables,
math, graphics, images, forms, etc): good idea, but removes chance of DTD
validation at the moment.
TEI? as we are one of the 4 master sites of the Text Encoding
Initiative (TEI), we leverage local expertise by using that.