[notriddle]

If you just want to have your language throw up an error whenever it sees an unrecognized element, then you’ll probably be able to simplify it a lot. It’s probably fine, since

* as long as you use a build tool the errors will be seen by the author (who will know how to fix them) and not the reader

* graceful degradation and format extensions are a crapshoot due to Hyrum’s Law [1]

[1]: https://news.ycombinator.com/item?id=30726668

[djedr]

Yes, this is probably the most sensible approach.

Other options would be to ignore everything you don't recognize or ignore all nodes of the form:

  unrecognized tag [value]

i.e. where you don't recognize the key and the value is not complex.

You could still display stuff like:

  unrecognized tag [
    [text]
    ...
  ]

i.e. text nodes which are children of unrecognized elements.

But approaches like these would need a good justification.