I wouldn’t say that it’d be strictly impossible, however if it can be done then it would come at a considerable cost to useability, versatility, etc.
One adjacent concept that comes to mind is the use of the :visited
CSS tag to extract a user’s browsing habits. I remember seeing a demonstration of this where an “are you human” captcha was shown but the choice of image in each box was controlled by the :visited
tag. I can’t find that post, but this medium article demonstrates a similer concept. There are mitigations to this luckily, but a fullproof solution would be to remove the tag’s functionality altogether, which would make certain websites (like the one we’re on right now!) much more inconvenient to use.
It seems trivial to me for a website to detect user behaviors that indicate the use of an adblocker. For example, if a request for a page is immediately followed by a request for a video on that page, rather than after 5-60 seconds, then they’re likey using an adblocker. If there is an ad placed between two paragaphs in an article, but two distant paragraphs are visible at the same time, it is more likely (although not guaranteed) that they are using an adblocker. If a user triggers an abnormal amount of those heuristics then they get flagged as an adblocking user.
I live in Washington state and I’m pretty certain the sales tax here is 10% (slightly higher than your maximum figure of 9.56%). It’s a pretty well known trick here that you can account for tax just by decimal shifting and adding (ex: 5.29$ without would be 5.29$ + 0.529$ ~= 5.81$ with tax). Is that 9.56% an “in practice” figure that accounts for rounding down? I’m curious where you read it.