It is a real problem for web designers. True ad's obviously do not want to identify themselves as ad's. Then they'd be easy to block. So, the software that tries to block ad's has to GUESS if something is an ad or not. It does this by looking at things like size of the graphic, position on the page, etc. When the software thinks it has found an ad, it removes it by actually deleting html text prior to passing it onto the display renderer. The rules the adblocker software uses continually evolve in order to keep up with the advertizers. A real mess.