I was thinking about XML, toml and JSON and I realised that they should be small. As long as they are small, they are readable and comprehensible.
Interestingly, XML scales a little bit better because it reminds you of the context when the context ends.
I really like some systems that incorporate inlining the code into their configs, but I'd prefer for it to go the other way — have configs in code.
The way #elixir and #webpack handle configuration is almost perfect.