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Yoav Weiss

@Matt Wilcox – Regarding the cookie issue, the AI cookie will not change, but in real life scenarios, the user might have other cookies as well (session ID, etc). “Vary: Cookie” means that the resource is invalid every time any cookie value is changed for this resource. If you have user specific cookies (again, session ID is the first one that come to mind), the resource is effectively non cacheable.
You could solve this by storing images on a cookie-less domain, different from your main domain. This is also a good practice in general, since it results in smaller requests for images. But, you can’t set a cookie on the images domain with javascript that runs in the context of the HTML, which is on the main domain.
All in all, it is complicated to get the images cacheable under the “URL must remain unchanged” assumption.