Ad blockers will block images that are standard banner ad size. Maybe a text ad has "ad" somewhere in it's in CSS. We do only internally hosted ads and non-JS text sidebar ads and they're happily blocked by all major ad blockers. Doing everything "right" is no defense.

I'm not saying you can't make ads completely indistinguishable from content but I also don't think that's the solution that we want either.

Some people just don't want ads. Paradoxically, the sites I care most about are the ones I want to periodically visit, and enjoy without the ads. So the ad blocker will have filters fine tuned for those.

Is that a sustainable business model? Well, no. There's a direct donation on sites nowadays luckily.

Thanks :-) I didn't know they also look for common banner sizes.

CSS classes and IDs match certain criteria, like being prefixed with "ad" or suffixed with "banner" and having a rendered size that matches an ad box (eg: 600px by 100px).

The ad blocklists are maintained by humans so they can block whatever elements that can be matched by the ad blocker's filter rules.