If we're baking file hashes into the HTML... is there any work out there that goes the extra mile and uses IPFS as your CDN?
A PWA with a Service Worker could perhaps implement its own client-side "gateway", translating public gateway URLs into direct IPFS access. Without the Service Worker (or without JS) it would fall back to using the gateway.
It sounds like we'd need to have browsers implement IPFS protocol support directly for this to be a feasible alternative to centralized CDNs.