I found hashicorp hcl after starting to think about what we wanted internally at PayWithMyBank.

The concepts are similar but we needed a language to change the composition and configuration of our screens that had the same kind of flexibility as the CSS language.

It is used as a replacement to XML/HTML to define visual components of the page, having a better structure, and to easily change the screen to different merchants, and to easily implement variations for A/B tests using CSS like rules.

Ex:

B { value = "1"; }

Defines B as having value = "1".

A > B { value = "2"; }

Define B as having value = "2" if direct child of A.

So the definition:

C [ B, A [ B ] ]

The first B will have value = "1" and the second value = "2".

Each definition act as a template (like XSLT) to create the elements.

Also there is a main difference that element composition is separated from element definition. So:

A { B { value = 4} }

Do not define the element A as having the element B. It define that if there is an element B having a parent A the value will be 4.

The language do not hard define the elements but define how to create the elements. You ask the language to create an specific element and all the rules will be evaluated to return the desired element.

Edited: Clicked enter before finishing the message.