https://html.spec.whatwg.org/multipage/embedded-content.html#list-of-available-images can be copied from document to document, but does not account for differences between such documents. E.g., they might have different CSP policies or one of them might disallow Mixed Content. We should probably only allow copying of this cache to take place if that does not break any of those security policies.

No idea if implementations handle this correctly today.