Mozilla experimented with the idea you're replying to -- a system that would download a bundle of disparate ads from a central server, and then decide based on

Mozilla experimented with the idea you're replying to -- a system that would download a bundle of disparate ads from a central server, and then decide based on information in the browser which ones to show. Take away active content from the ads, or (very distant second best) sandbox them, and you can do targeting without the advertisers or even the distributor being able to figure out why a particular ad was shown.