New IETF RFC (approved around September 2025) that lets a client use a resource it has already downloaded as a Brotli/Zstandard dictionary for compressing a subsequently downloaded resource. Turns a new release's JavaScript delta into a tiny patch (bytes instead of re-downloading the whole file) and can be used as 'standalone dictionaries' to diff away shared header/footer HTML across pages — effectively replicating the SPA shell-plus-content model without JavaScript. Wire format is a couple of HTTP headers (DCZ for Zstandard, DCB for Brotli); server needs to key stored dictionaries by hash and apply compression per request. Ryan measured ~96% compression ratio on his personal site (20KB → 5KB). Progressive enhancement — no reason not to deploy today for Chrome users. Firefox had a canary implementation rolled back for bugs; Safari has started pulling tests into WebKit.