The linked spec in the original has all the hallmarks of "oh wait, maybe we do need...".

It reads like what you get when someone has a poor idea of what 2D UI requirements are (alignment, relative positioning, layout, out-of-band image delivery, updates, animation, ...) and then has to retroactively fit them in one by one.

And then the mechanisms it has for doing so read like they are a poor fit for how you want to use them, requiring weird global bookkeeping/state, and sound like a nightmare without a proper API around it, abstracting it away.

Aka reinventing all the lessons of 30 years of UI development from scratch.

From what I remember sixel is _worse_ but that doesn't really change things.