I’ve become ambivalent about relaxng, I used it a bunch because I like(d) the model, and the “Compact” syntax is really quite readable, and it’s a lot simpler than XML schemas.

However the error messages, at least when doing rng validation via libxml2, are absolutely useless, so when you have a schema error finding out why tends to be quite difficult. I also recall that trying to allow foreign schema content inside your document without validating it, but still validating your own schema, is a bit of a hassle.

the compact non-xml syntax is neat: https://relaxng.org/compact-tutorial-20030326.html#id2814005

it reminds me of TypeScript.

As for XML itself, it seems like it was a huge buzzword fad in the late 90s/early 2000s, but it must not have lived up to the hype or we’d actually be using it today instead of JSON and Protobuf. I got to computer programming around when the web gave up on XHTML, so i’m not really sure what to make of the XML cultural moment. The vibe i get is of focus on precise data semantics for its own sake, very Cathedral, effort that didn’t end up delivering benefit to humans. What do you think?