I'd really like a collection of unit tests for parsers. There is a lot of details that can differ between parsers.
E.g. in "Section C" the resulting KeyThree is "value 3▵▵▵▵▵▵▵value 3 continued" where each "▵" symbol is a space.
I think most people would expect a single (or no) space here.
I would guess that most software would strip the comments in SectionC or rearrange the output so that it will result in a diff even when nothing in SectionC changed.
So if you edit the file by hand in the same style as shown in the examples, then most editors would not be able to make a minor edit without making a large diff as many sections would be formatted differently.
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.
https://www.freedesktop.org/software/systemd/man/latest/syst...