accessibility is not a behavior that "people liked". It is enshrined in law in so many countries that they _have_ to be at the lowest level of the system.
I would argue it was enshrined in law because people liked it so much they demanded it, so maybe that's a semantics issue? That said, I do not believe the law insists that it be in the lowest level: the law merely insists that it exist for every site. It is convenient to put it at the lowest level if possible, but now it is just one more thing that literally every browser has to implement from scratch, and so the world that results is that the only company capable of making a good web browser is Google. There are sane reasons why accessibility is not considered part of a CPU (and thereby by most other reasonable runtime engines).
People with reading disabilities, vision issues, etc are rarely the ones being heavily represented in parliaments. Accessibility is truly a case of things being earned through spilled blood, and while there's probably much fewer people dying because they can't read the text on your website, it's a natural extension of the physical world. While the laws do not say that it should be up to the browser, it becomes natural that this moves to the lowest common denominator. Should it not be part of browsers, the moment any dominant browser comes in (say, Chrome) and implements it would lead to thousands of websites relying on this behavior from the browser, leading to "Works only on Chrome" situations, and other browsers most likely rushing behind to also support that feature.