Because this is running on the customer's browser, CPU efficiency doesn't matter at all. They can do all sorts of stupid things, like grab the DOM from the browser, lay it out in a JS rendering engine and see if their div ends up being visible.

I haven't had the time to look through the ad javascripts on your typical fat webpage, but when there's tens of megabytes of code and it pegs my CPU at 100%, I know it's doing more than just displaying a .png in an iframe.

This kind of resource misuse is one more reason people run adblockers.

That and the security risks of running other people's code on your system.

Could you just pick a couple of pixels from the actual page display and check they're the right color?

Not sure either of these work without access at the top page level, ie you can't get the data from scripts in an iframe? Been a while since i did any JS dev.