Do you have a writeup of the technical details? How does this exploit (without reading the code and reverse engineering it)?

They did. I think the idea with XML based hypertext was to have links that are star-shaped with potentially multiple sources and potentially multiple targets and the location where the link is defined completely independent from any sources or targets.

One application would have been that people publish link collections and you could e.g read HN with the links I created.

There was a proposed standard for this but its name escapes me for the moment.

Of course all of this never went farther than XHTML where it took a sharp turn into a different direction.

Sounds like a fun item for an escape room.

Does a proof of concept need a use case? :)

Take a menu in a tourist spot, could have it go to both languages

real-world AB testing is the only thing I can think of

Not all questions have an answer that will satisfy you.

In this case, it probably exists because it's technically interesting, and for no other reason.

Although I can totally image a roulette style game played by people who like that kind of thing: one link leads to something nice like puppy photos, the other leads to something disgusting.