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)

Automerge is a local-first sync engine for multiplayer apps that works offline, prevents conflicts, and runs fast.

How to build the local-first software with the most interoperable data system: files.
How to avoid conflict while syncing them on cloud providers? Tonsky relates some strategy.

A collection of resources such as guides, blog posts, advocacy, how to's

IndexedDB can be used to store a lot of data. It has some caveats though.

Storage:
About deletion, use soft delete to smoothen the synchronization if a user deletes a record and another one update it.

About record collection, use unique IDs (UUID v4) or property related ids with (UUID v5).

About ordering, it is easier to use fractional indices! Read more on https://www.figma.com/blog/realtime-editing-of-ordered-sequences/, or https://www.steveruiz.me/posts/reordering-fractional-indices, or use a dedicated library.

Sync is made with pull and push
Update:

• Send atomic changes from a client is the more convenient way. We can send only the model’s ID and its updated fields.
• send operations instead of changed data,

Conflict resolution:

• In some cases, last-write wins at the record field level will be enough
• in others, we strongly need a full-fledged CRDT.