The author uses a service worker in wasm to render HTML. The service worker syncs the data with the server.

Note fetch requests can be intercepted with the ... fetch event in service workers

The service worker used here is written in Go. Note the "localfirst" approach runs only after the service worker is loaded. The initial page is loaded as simple HTML because of SSR. That's the advantage of WASM: the code runs on the client and the server.

(following https://shaarli.lyokolux.space/shaare/CosnyQ)