To preserve backwards compatibility and not require all those old sites to update, the legacy behavior would have to be the default, with opt-in for the new behavior.

▲

mejutoco 6 hours ago | parent [-]

That is the opposite approach. Also an option. One could also deprecate the call without parameter and force always a parameter with which behaviour. The deprecation could last enough time that those websites would have been rewritten multiple times ;)

	▲	sfink 5 hours ago \| parent [-]
		The control interface burned into your hardware device will not have been rewritten. And it's not like you can have a flag day where everyone switches over, so the lifespan of those hardware devices isn't that relevant. Backwards compatibility is a large part of the point of the Web. You could version everything at whatever granularity you like, but over time that accumulates ("bug 3718938: JS gen24 code incorrectly handles Date math as if it were gen25-34", not to mention libraries that handle some versions but not others and then implicitly pass their expectations around via the objects they create so your dependency resolver has to look at the cross product of versions from all your depencies...)