DocBook is a semantic markup language used to both define documents as structured data (and its elements ) along with explaining the meaning of those data (and its elements.)
The domain of the documen structured data is completely open and unlimited meant for 100% definition by the user.
It publishes to just about everything.
An open standard created and maintained by self-sacrificing volunteers and BIG companies it is beloved, high-quality, and suffers the same ills and pangs of all human endeavors