You should read some of James Clark's criticisms of the official XML Schema (XSD) standard, which motivated him to develop TREX (Tree Regular Expressions for Xml), which he combined with Makoto Murata's RELAX (REgular LAnguage description for XML) to create Relax/NG.
https://en.wikipedia.org/wiki/James_Clark_(programmer)
https://en.wikipedia.org/wiki/Makoto_Murata#RELAX_and_RELAX_...
>Some people, including Murata and James Clark, had critical attitudes toward XML Schema. XML Schema is a modern XML schema language designed by W3C XML Schema Working Group. W3C intended XML Schema to supersede traditional DTD (Document Type Definition). XML Schema supports so many features that its specification is large and complex. Murata, James Clark and those who criticised XML Schema, pointed out the following:
>It is difficult to implement all features of XML Schema.
>It is difficult for engineers to read and write XML Schema definitions.
>It does not permit nondeterministic content models.
>Murata and collaborators designed another modern schema language, RELAX (Regular Language description for XML), more simple and mathematically consistent. They published RELAX specification in 2000. RELAX was approved as JIS and ISO/IEC standards. At roughly the same time, James Clark also designed another schema language, TREX (Tree Regular Expressions for XML).
>Murata and James Clark designed a new schema language RELAX NG based on TREX and RELAX Core. RELAX NG syntax is the expansion of TREX. RELAX NG was approved by OASIS in December 2001. RELAX NG was also approved as Part 2 of ISO/IEC 19757: Document Schema Definition Languages (DSDL).
https://en.wikipedia.org/wiki/Regular_Language_description_f...
https://en.wikipedia.org/wiki/RELAX_NG
https://en.wikipedia.org/wiki/XML_Schema_(W3C)
Schema Wars: XML Schema vs. RELAX NG (1/2) - exploring XML
https://web.archive.org/web/20180429143242/http://webreferen...
https://web.archive.org/web/20180429145711/http://webreferen...
https://news.ycombinator.com/item?id=22756875
>James Clark used Haskell to design and implement an algorithm for validating Relax NG XML schemas (he co-designed Relax NG, and designed its predecessor TREX), to work the ideas out before re-implementing it in (many many more lines of tedious brittle) Java (JING). Haskel works wonderfully as a design and standard definition language, that way.
https://news.ycombinator.com/item?id=25435678
>James Clark's compact syntax for Relax/NG XML schema validation language is quite tastefully designed, an equivalent but more convenient alternative syntax than XML, for writing tree regular expressions matching XML documents. It's way more beautiful and coherent than the official "XML Schema" standard.
[...]
>There's a wonderful DDJ interview with James Clark called "A Triumph of Simplicity: James Clark on Markup Languages and XML" where he explains how a standard has failed if everyone just uses the reference implementation, because the point of a standard is to be crisp and simple enough that many different implementations can interoperate perfectly.
>A Triumph of Simplicity: James Clark on Markup Languages and XML:
https://web.archive.org/web/20130721072712/https://www.drdob...
"The standard has to be sufficiently simple that it makes sense to have multiple implementations." -James Clark