> a user agent is not forced to follow the instructions
Luckily! A large german media corporation called "Springer" has for years, and is still, unsuccessfully trying to get the courts and politicians to rule that users can not manipulate web content and must run it as intended, as changing it would violate copyright and is a sabotage of their program. And i bet they aren't the only ones globally. Also: how many devices are locked down and can only run code as it is provided by trusted third parties? Try installing an ad-blocker on a smart-tv or a playstation.
Luckily! A large german media corporation called "Springer" has for years, and is still, unsuccessfully trying to get the courts and politicians to rule that users can not manipulate web content and must run it as intended, as changing it would violate copyright and is a sabotage of their program. And i bet they aren't the only ones globally. Also: how many devices are locked down and can only run code as it is provided by trusted third parties? Try installing an ad-blocker on a smart-tv or a playstation.