My entry on web adblockers and the Usenet killfile problem sparked a discussion on lobste.rs, and as part of that discussion I came to an obvious in retrospect realization about why adblockers are so effective and are going to stay that way. Namely, it's because of the needs of modern ad networks.

The modern ad-laden web page is assembled on the fly in the browser, through a variety of methods for getting client-side included content; JavaScript, embedded iframes, images loaded from outside domains, and so on. This client side assembly is essentially forced on the web ad ecology because ads are both widely distributed and heavily centralized. The ecology is widely distributed in that ads appear on a huge number of websites, large and small; it is heavily centralized because modern ad networks want to be as big as possible, which means being able to rapidly place ads on a lot of websites.

If an ad network wants to be big, it generally has to have a lot of websites that will display its ads. It can't afford to focus all its efforts on a small number of sites and work deeply with them; instead it needs to spread widely. If you want to spread widely, especially to small sites, you need to make it easy for those websites to put your ads on their pages, something simple for both them and you. Having the ads added to web pages by the browser instead of the web server is by far the easiest and most reliable approach for both the ad network and the websites.

(Adding the ads in the client is also kind of forced if you want both sophisticated real time ad bidding systems and web servers that perform well. If the web server's page assembly time budget is, say, 30 milliseconds, it isn't going to be calling out to even a single ad network API to do a 100 millisecond 'bid this ad slot out and give me an ad to serve' operation.)

When you leave it up to the web browser to add the ads to the web page, you make it possible for the web browser to turn around and not actually do that, fundamentally enabling adblockers. When you operate at scale with simple-to-add, simple-to-support snippets of HTML, JavaScript, or CSS that create the hooks for this assembly process, you need exactly the generic 'this thing will be an ad' markers that make it easy for adblocks to block this content en masse across many sites.

Big sites can do artisanal content management systems that integrate ads into their native page content in their backends, serving the entire thing to you as one mostly indistinguishable mass (and they're big enough to make it worthwhile for ad networks to do custom things for them). But this is never going to happen for a lot of smaller sites, which leaves ad networks creating the environment that allows adblockers to flourish.