> Actually, it makes me wonder what's keeping these companies that want to serve ads generate JS dynamically server side and include the code for ads in the sam

> Actually, it makes me wonder what's keeping these companies that want to serve ads generate JS dynamically server side and include the code for ads in the same file as the main application logic. First reason is the lack of trust. Or limited trust, if you prefer. That is, SomeCompany may trust AdProvider enough to include their ads in its website, but then again not trust them enough to bundle random external code with their own. Letting the user's browser load from different origins does give you some separation. Then there's the separation of responsibilities. Say something does break -and not because of blocking-, having the code bundled together makes it very easy to start a game of passing the blame. "It works for us", "We're not doing anything weird", etc. And not only that. There's also the question of who pays for that downtime. SomeCompany is kept from earning money, will AdProvider compensate them? SomeCompany'll probably need to provide evidence that the problem comes from AdProvider. On the other hand, some websites may already be doing a very similar thing... though it's likely just accidental. I mean, I have experienced a fair number of websites breaking when googleanalytics is blocked and I'd bet it's not intentional, just bad programming.