Browsers have local storage (limited to 4mb, key value store), indexedDB and some browser also have file system apis. google made great progress in making a WASM sqlite port performant. So I think we just need to look into how we can link it to the rust core. https://developer.chrome.com/blog/sqlite-wasm-in-the-browser-backed-by-the-origin-private-file-system
The serious discussion for making this work is on https://support.delta.chat/t/what-would-be-needed-for-a-standalone-web-version-without-a-server-component/3789, if you want.