nanoSQL looks very interesting. I've had a few requirements for more complex data structures in the browser and key/val systems don't work.
NanoSQL gives me a sort of query language for local storage (can be indexeddb, can be websql, the adapter can decide).
Claims:
Universal database layer for the client, server & mobile devices. It's like Lego for databases.
Not so sure about "Lego for dbs", but certainly going to give it a try.
Source: nanosql.io