I had anticipated the overhead of XML being brought up, hence the other suggestions there, which is anything + jsonschema. jsonschema was used for illustrative purposes, it's a lot more powerful than most usecases call for, and it pays forward for that by making the syntax very verbose and longwinded. It'd certainly be an alternative with less overhead, though.

I've not benchmarked XML parsing times in a very long time, I'd be interested in seeing the numbers now.

Tabular data is barely data, honestly, hence my PS, I don't think spreadsheets in general are a very good way to store anything.

And yes, absolutely, overengineering is bound to happen, which is unfortunate, but I'm not sure if it really can be avoided while still keeping many of those upsides (especially the rigorous definitions)