I think part of the data format could be visual related tho. Enough to make sites able to look distinctive, but not enough to allow the chaos we have today. For example, background image for a page, font for a page, whether something should be in a left panel, center panel, or right panel. They would be hints and very limited, and all in one place, in the data format. Unlike HTML where you have anything, anywhere, and total chaos.

I think you underestimate the value the world would place on the gains we could get from this kind of design, not to mention that AI would be able to reason about the pages much better. Just the AI angle along is enough to convince society to go along with the change.

But there will need to be some "Killer App" that does this, which everyone else copies.