> Also, on some browsers, checkboxes don't even have the lauded accessibility you're thinking about
In which case, people with accessibility needs won't be using that browser. And that's the whole point. The user can pick whichever browser works best for them, and implements the standard controls in the way they want. (Or, in the way that sucks the least, out of the browsers available to them.) And if they decide to get a better browser (different engine, or even just a major upgrade to the current one) suddenly all their websites work better. More importantly, they all work better in exactly the same way, so they only have to get used to any differences once, rather than for each and every website.
If you go and write your own version of a standard control which implements it that way you want, you take that away from the user.
HTML allows you to specify the semantics of the controls you want, and have the user-agent bother with the details about how it works. That's going to be more lightweight, more performant, and more user-friendly for more people than any alternative you bodge together yourself.
In which case, people with accessibility needs won't be using that browser. And that's the whole point. The user can pick whichever browser works best for them, and implements the standard controls in the way they want. (Or, in the way that sucks the least, out of the browsers available to them.) And if they decide to get a better browser (different engine, or even just a major upgrade to the current one) suddenly all their websites work better. More importantly, they all work better in exactly the same way, so they only have to get used to any differences once, rather than for each and every website.
If you go and write your own version of a standard control which implements it that way you want, you take that away from the user.
HTML allows you to specify the semantics of the controls you want, and have the user-agent bother with the details about how it works. That's going to be more lightweight, more performant, and more user-friendly for more people than any alternative you bodge together yourself.