This is pretty much never done server-side. It would unnecessarily increase server load by requiring callbacks and/or polling. It’s always done in JavaScript or, for really simple setups, CSS.
Nobody running ads is considering users who block JavaScript wholesale. Not only is it a trivially small percentage of users, but it’s also a waste of time: those users aren’t going to stop blocking JavaScript to display ads anyway. Instead, they’ll just make it so the site fails to render at all if JavaScript is disabled.
You’ve presented a solution looking for a problem.
As for anti theft gizmos, they’re talking about the alarm tags and ink packs that get attached to merchandise so it sets of an alarm when you try to leave with it, or the item is covered in ink when you forcibly remove the security tag. These are passive anti-theft security and do not constitute spying.
Even if we were talking about video cameras, it’s unreasonable to call that “spying”. In order for something to be spying you need an expectation of privacy, which is not present when you’re inside a private business. It would be like calling a security who observes customers a spy. It’s absurd.
This is pretty much never done server-side. It would unnecessarily increase server load by requiring callbacks and/or polling. It’s always done in JavaScript or, for really simple setups, CSS.
Nobody running ads is considering users who block JavaScript wholesale. Not only is it a trivially small percentage of users, but it’s also a waste of time: those users aren’t going to stop blocking JavaScript to display ads anyway. Instead, they’ll just make it so the site fails to render at all if JavaScript is disabled.
You’ve presented a solution looking for a problem.
As for anti theft gizmos, they’re talking about the alarm tags and ink packs that get attached to merchandise so it sets of an alarm when you try to leave with it, or the item is covered in ink when you forcibly remove the security tag. These are passive anti-theft security and do not constitute spying.
Even if we were talking about video cameras, it’s unreasonable to call that “spying”. In order for something to be spying you need an expectation of privacy, which is not present when you’re inside a private business. It would be like calling a security who observes customers a spy. It’s absurd.