The threat model of one site leveraging the user's browser to covertly and maliciously engage with a third-party site was something that emerged and matured gra

The threat model of one site leveraging the user's browser to covertly and maliciously engage with a third-party site was something that emerged and matured gradually, as was the idea that a browser was somehow duty-bound to do something about it. Browsers were just software that rendered documents and ran their scripts, and it was taken for granted that anything they did was something the user wanted or would at least hold personal responsibility for. Outside of corporate IT environments, users didn't expect browsers to be nannying them with limitations inserted by anxious vendors and were more interested in seeing new capabilities become available than they were in seeing capabilities narrowed in the name of "safety" or "security". In that light, being able to acccess resources from other domains adds many exciting capabilities to a web session and opens up all kinds of innovate types of documents and web applications. It was a long and very gradual shift from that world to the one we're in now, where there's basically a cartel of three browser engines that decide what people can and can't do. That change is mostly for the best on net, when it comes to enabling a web that can offer better assurances around access to high-value personal and commercial data, but it took a while for a consensus to form that this was better than just having a more liberated and capable tool on one's computer.