Can we agree that some webpages and webapps have already make the decision to control the locale through the app and not choose the environment default?
My OS and browser locale is pt-BR, but if for whatever reason I am not happy with a pt-BR UX, which is often, I want to be able to change it on a case by case basis.
For example, any documentation I look up, like say on MDN or AWS, I want to see it in english and not a poor/incomplete translation. If I play a Star Wars game on my browser or phone I do not want any translations, I want the original terms like scoundrel instead of the translator's choice of terms.
It is already bad enough as it is for the reasons I pointed out, now we are gonna have that same nightmare in forms as well?
Sure webapps that actually care will find a non standard way, as they already do, but then we are not actually improving when in 2017 you have to load 3rd party libs just to be able to choose the locale of a date input.
<input type=date> (environment default)
<input type=date locale="pt-BR"> (actual chosen locale)
Is that too much to ask?