My estimate of 80% included 6-12 months of serious development first, and a certain amount of budget for manual intervention for the first several dozen jobs. Certainly just flinging HTML at ChatGPT as it stands today would do nothing useful at all. Providing manual testing could easily be done as part of a higher service plan. Not only is there no rule that a startup using AI
has to be just in the form of "throw it at the AI and then steadfastly refuse to do anything else", that's probably a pretty good way of filtering out the ones that will make it from the ones that won't.
Do assistive technologies have more "bugs" and "quirks" and "different behaviors" than natural text? I don't really think so. In fact I'd expect they have qualitatively fewer such things.
Semantic HTML would be important in this case... but it would be important as the output, not the input.
This hypothetical startup could also pivot into developing a better screenreader pretty easily once they built this, but there would be a good few years where an AI chewing on the HTML and HTML templates in use by a server would be practical but you can't expect every assistive technology user to be using a 64GB GPU to run the model locally. Certainly that would factor into my pitch deck, though.
I'd give more credence to the "it has to be perfect to be useful at all" argument you're going with here if it weren't that I'm pretty sure every user of such technology is already encountering a whole bunch of suboptimal behavior on almost every site today.