My application [0] I used daily for 13 years stores the data in localStorage and I had several unpleasant data loss incidents - mostly me cleaning storage of the wrong browser tab. Thankfully, a simple JSON dump/import feature enables a good enough degree of reliability. Thanks to it, I didn't lose any data in 5 years, but now I have a questionable habit where I defensively export data before every major edit or deployment.
Simple localStorage replication has been an interest of mine for many years and I explore available solutions multiple times a year, but none of these ticks all the boxes - no server to maintain; free; optional for users; bring your own storage; the app stays a web page; cross-browser. I wish browsers had a file system API that allowed the app to read/write to specified directory or a file in background, which users may sync with their tool of choice, like Google Drive.