[Ajedi32]

And to be clear when I say "a regulation like this" I'm referring to the one about adding emergency alerts to web browsers. That's an arguably useful feature for browsers to have, and it's probably not a big deal if some open source browsers don't have it so full enforcement isn't necessary.

Things get way harder when you're talking about forcing the addition of user hostile features (like the bill described in the OP presumably is; blocking users from accessing a website that they want to access), because that creates a situation where vendors have to treat the device owner as a potentially hostile actor. I don't see any way you could enforce that without completely destroying users' freedom to run software of their choosing on their own device, no matter how the law is worded, and I wouldn't consider that "reasonable" under any circumstances.