The original designers of YAML explicitly declared in its name "Yet Another Markup Language" that it was designed to be a markup language, which it most certainly was not. Eventually somebody finally pointed out the mistake, and they sheepishly retronymed it "YAML Ain't Markup Language".

https://stackoverflow.com/questions/6968366/if-yaml-aint-mar...

On the other hand, and to your points, Relax/NG (both its XML and simplified syntaxes) is a beautiful successful example of wisely and collaboratively designing a new clean powerful system with the deep understanding of what a markup language really is (James Clark was deeply involved with many SGML and XML standards and implementations), and full cognizance of the strengths and weaknesses of other systems you're trying to replace (SGML DTDs, XML Schemas, TREX, RELAX, XDuce, and other experimental XML schema languages).

https://en.wikipedia.org/wiki/RELAX_NG

>In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML—a RELAX NG schema specifies a pattern for the structure and content of an XML document. A RELAX NG schema is itself an XML document but RELAX NG also offers a popular compact, non-XML syntax.[1] Compared to other XML schema languages RELAX NG is considered relatively simple.

RELAX NG Compact Syntax

https://www.oasis-open.org/committees/relax-ng/compact-20021...

https://relaxng.org/jclark/design.html

The Design of RELAX NG

James Clark (jjc@thaiopensource.com)

Abstract: RELAX NG is a new schema language for XML. This paper discusses various aspects of the design of RELAX NG including the treatment of attributes, datatyping, mixed content, unordered content namespaces, cross-references and modularity. [...]

>Composability

>RELAX NG is designed to be highly composable. A schema language (or indeed a programming language) provides a number of atomic objects and a number of methods of composition. The methods of composition can be used to combine atomic objects into compound objects which can in turn be composed into further compound objects. The composability of the language is the degree to which the various methods of composition can be applied uniformly to all the various objects of the language, both atomic and compound. For example, RELAX NG provides a choice element that can be applied uniformly to elements, attributes, datatypes and enumerated values. This is not mere syntactic overloading. The choice element has a single uniform semantic in all these cases and can have a single implementation. Another example is the grammar element, which is the container for definitions. The grammar element is just another pattern and can be composed in just the same way as other patterns. Composability improves ease of learning and ease of use. Composability also tends to improve the ratio between complexity and power: for a given amount of complexity, a more composable language will be more powerful than a less composable one. [...]

>XML syntax

RELAX NG uses XML instance syntax to express schemas. Although this makes for a rather verbose schema language, it has some major advantages. Since a user of an XML schema language must necessarily already learn XML instance syntax, using XML instance syntax for the schema language reduces the learning burden on a schema user. It also allows XML tools and technologies to be applied to the schema. For example, a schema can be used to specify the syntax of the schema language. Another important benefit of XML syntax is extensibility. RELAX NG has an open syntax that allows the RELAX NG defined elements and attributes to be annotated with elements and attributes from other namespaces. RELAX NG DTD Compatibility [12] uses this annotation mechanism to extend RELAX NG with a mechanism for declaring default values for attributes. RelaxNGCC [23] uses this annotation mechanism to allow users to embed Java code in RELAX NG schemas, which gets executed as an XML document is parsed against the schema. An unofficial non-XML syntax for RELAX NG has also been developed [8]. The non-XML syntax can be used for authoring RELAX NG schemas by hand and can then be transformed into the standard RELAX NG XML syntax for interchange. [...]