(recommended lib https://github.com/mozilla/localForage, or Dexie(http://dexie.org/), use IndexedDB for browsers support it,
fall back to WebSQL (like safari), then localstorage for basic support browser)
- IndexedDB
- localStorage
- Application Cache
- XMLHttpRequest
- offline detection mechanisms / library (offline.js)
- use `indexedDB` for complex web app offline asset storage
- `serviceworker` will be a better solution for offline asset storage and use.