I've been using TeX for a long while, and one thing which has frequently struck me is that documents are fundamentally a key-value/markup-content pair and it re

I've been using TeX for a long while, and one thing which has frequently struck me is that documents are fundamentally a key-value/markup-content pair and it really should be much easier to to markup and tag documents, and then once they are tagged, changing the appearance should be as simple as swapping out a .css file (which can be achieved in TeX, but requires a bit of effort, though newer documentclasses such as memoir make this much easier), but there doesn't seem to be any sort of effort to stuff a document into a database, re-work it, then get a finished product out. Jupyter Notebooks seem a step in the right direction, but there's no provision for the appearance AFAIK.