If it were me, I'd make third-party font sources require a SHA hash. In pseudocode: url("https://fonts.googleapis.com/comic-sans", sha="abcd1234") This way: - If my browser has comic-sans cached, no request is made - Caching works even if the same resource is sourced from multiple places (e.g. I can host comic-sans locally, but if they got it from a CDN, they don't need to get it again) - If a malicious site replaces…
One issue with cross-site caching, though, is that it may enable timing-based attacks on privacy.