[otabdeveloper4]

Something can be "not about UI" and "the most successful UI platform in the world" at the same time.

Many such cases. :)

Point is, HTML is a document format, not a UI toolkit. That's how it was designed and that's how it continues to evolve.

This is probably not a ideal situation, but trying to change it now probably won't do any good.

[contravariant]

Well like it or not but the rise of REST APIs suggests that HTTP is extraordinarily well suited to use as a general interface so it's not too surprising that the related markup language HTML has found similar use as a user interface.