Not sure if this is the right SE, feel free to migrate if necessary.
My question is simple: the only way for a web server to detect if you are adblocking is to see if the ads were actually served, since once they're client-side it's trivial to just not show that part of the html. Right?
I understand why adblocking software doesn't download the ads: it would be very wasteful. But what I don't get is, for sites like Forbes that force you to turn off your adblocker, why not just have a mode where you can download the ads anyway and just not show them? As opposed to having to turn off the adblocker.
If I download the ads but don't display them, is there any way the web server could detect I'm adblocking? Like with client-side JS or something? What are the theoretical limits and practical limits on adblocking?