Since systemd is successfully parsing its INI files, and barks at you when you put weird shit into them, a grammar for them does exist as well.

XML is that wonderful format that gave us vulnerabilities like death by million laughs, up to a certain moment, you could MitM DTDs, and a whole slew of everything-XML stuff back when XML was like AI is today, none of which I miss today.

Oh, and remember times when programmers would argue whether argument order in XML files should be significant or not?

But XML books with their idealized XML future description did give me the same warm fuzzies as some intricate clockwork mechanism to a Victorian geek.