Site A can't see the cached resources of site B, because it would show that the user has visited site B if the resource is provided too fast. However, if the resource is provided with some delay, then site A assumes that site B hasn't been visited.
The current solution is to download the same thing twice. It is wasteful and I offer downloading it only once, but if site A asks for a resource for site B, the resource should not be returned immediately, but after a few ms so that site A gets the wrong conclusion.
It should work if site A can observe only the result of the request but can't check if a request happened or not.
Ah, right, I understand your point now. When sites determine if something is cached or not, are they doing it purely based on the time taken to load it?
The current solution is to download the same thing twice. It is wasteful and I offer downloading it only once, but if site A asks for a resource for site B, the resource should not be returned immediately, but after a few ms so that site A gets the wrong conclusion.
It should work if site A can observe only the result of the request but can't check if a request happened or not.