I made BML for that purpose. I posted about it before: https://news.ycombinator.com/item?id=8645591

To copy-paste the example:

    server
      path: /core/www/
      host: example.com
      port: 80
      service: true
      proxy
        host: proxy.example.com
        port: 8080
        authentication: plain
      description
        :Primary web-facing server
        :Provides commerce-related functionality
    
    server
      ...
      proxy host="proxy.example.com" port=8080
        authentication: plain

There's no character escaping, brackets, distinction between attributes and elements, data types (up to the one parsing the document how to decode values), etc. BOM-free UTF-8 is mandatory.

It's the bare minimum for an extensible tree-based data structure, but I threw in three ways to assign values to nodes to help craft smaller documents. Turns out it's hugely important to be able to assign multiple child nodes on the same line as the parent for certain classes of documents.

My implementation of BML uses my string library, so I can't offer a portable parser. But it shouldn't take more than an hour or two to write a BML parser.

Still, my reference example is here: https://gitlab.com/higan/higan/tree/master/nall/string/marku... -- 177 lines for the base parser, which can also save back modified BML documents.

Downside is that I have no clue how to market it, and I hate writing documentation, so there's only a few people using it.