Other languages have failed at this goal (because they were really designed for something else)

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RDF also tried to let webpages make their metadata public.

it's designed for librarians and techies: too complex, and has some limitations

XTM (Topicmaps) does the same stuff as XFML, but better, and a lot more stuff.

But it's not easy! Ease matters.

XFML is designed so it is easy to implement code for it, and easy to work with for real people.

XFML is focussed on one goal (a big enough goal for one language).

Topicmaps (XTM) are meant to solve far more difficult problems than XFML. Using XTM for the simple stuff would be overkill (which is why nobody does it).