The Web SQL Database specification is no longer being maintained and support may be dropped in future versions.

Also - these fallbacks are not exactly swappable. Its like swapping from a Database to a Filesystem as a fallback. Difficult to abstract into a single library/api.

And you still potentially leave out IE users if those are your options (depending on your IndexedDB needs).

IndexedDB to localStorage fallback might work - but again might not be an easy API abstraction.