There are actually several layers of "cache" inside of a browser, including the traditional HTTP cache (which is global) as well as the site storage, which includes stuff like IndexedDB, WebSQL, LocalStorage, AppCache, and window.caches (all of which is per origin).
This site storage _can_ be cleared by the browser (it's "temporary" per the spec), but in practice it isn't very frequently cleared unless the machine is running low on space. E.g. Chrome only does it if a site exceeds 20% of total per-origin browser storage (https://developer.chrome.com/apps/offline_storage) whereas Edge is extremely conservative with clearing IDB because it's considered user data (e.g. email drafts in Outlook). In any case, when the browser does clear this storage, it clears everything at once for that origin, so the user essentially has the experience of visiting the site for the first time. This is why it's a good practice to periodically sync your PouchDB data to CouchDB because it can be lost in rare cases.
Also there is a new Storage spec that allows site authors to designate certain buckets of per-origin storage to be persistent, but this typically requires a user permission and isn't widely supported yet: https://storage.spec.whatwg.org/
There are actually several layers of "cache" inside of a browser, including the traditional HTTP cache (which is global) as well as the site storage, which includes stuff like IndexedDB, WebSQL, LocalStorage, AppCache, and window.caches (all of which is per origin).
This site storage _can_ be cleared by the browser (it's "temporary" per the spec), but in practice it isn't very frequently cleared unless the machine is running low on space. E.g. Chrome only does it if a site exceeds 20% of total per-origin browser storage (https://developer.chrome.com/apps/offline_storage) whereas Edge is extremely conservative with clearing IDB because it's considered user data (e.g. email drafts in Outlook). In any case, when the browser does clear this storage, it clears everything at once for that origin, so the user essentially has the experience of visiting the site for the first time. This is why it's a good practice to periodically sync your PouchDB data to CouchDB because it can be lost in rare cases.
Also there is a new Storage spec that allows site authors to designate certain buckets of per-origin storage to be persistent, but this typically requires a user permission and isn't widely supported yet: https://storage.spec.whatwg.org/