I find that YAML can be, if stripped down to the nice parts, a delightful config file structure.

Here's the toml.io example in perfectly legal YAML but using only YAML's nicer, JSON-like, compatibility grammar:

    title: "TOML Example"

    owner: {
        name: "Tom Preston-Werner",
        dob: 1979-05-27 07:32:00 -08:00
    }

    database: {
        enabled: true,
        ports: [8000, 8001, 8002],
        data: [
          ["delta", "phi"],
          [3.14]
        ],
        temp_targets: { cpu: 79.5, case: 72.0 }
    }

    servers: {
        alpha: { ip: "10.0.0.1", role: "frontend" },
        beta: { ip: "10.0.0.2", role: "backend" },
    }

If I were on the YAML board (?) I would push this or a similar subset (JAML? DUML? DUMBL?) to be implemented by parsers in every language: yaml.parse(yamlString, { jamlMode: true }). But it already works today anyway if you stick to the format. And that's what I use for my apps.

Multi-line strings in YAML are also very similar to TOML and you can ignore all different character mixups and stick to '|' (w/ newlines) and '"' (no newlines).

    # same as " hello world! "
    str1: "
        hello
        world!
      "

    # same as "apples\noranges\n"
    str2: |
      apples
      oranges

Most numeric TOML examples work too, minus a few bells and whistles like numeric separators '_':

    # integers
    int1: +99
    int2: 42
    int3: 0
    int4: -17

    # hexadecimal with prefix `0x`
    hex1: 0xDEADBEEF
    hex2: 0xdeadbeef

    # octal with prefix `0o`
    oct1: 0o01234567
    oct2: 0o755


    # fractional
    float1: +1.0
    float2: 3.1415
    float3: -0.01

    # exponent
    float4: 5e+22
    float5: 1e06
    float6: -2E-2

    # both
    float7: 6.626e-34

    # infinity
    infinite1: .inf # positive infinity
    infinite2: +.inf # positive infinity
    infinite3: -.inf # negative infinity

    # not a number
    not1: .nan