There have been projects to add Couch semantics to browser storage however I do not think any are production-ready. Progress usually halts when it's time to implement the replication protocol. It's not rocket surgery, however as I will say in another comment, the only "documentation" is the Erlang reference implementation.
However, the situation you describe is pretty much the primary objective of much of the CouchDB leadership, so I'm optimistic that this will happen. In the meantime, people simply run CouchDB on the local device or even as a browser plugin. Ubuntu does that, and CouchDB can be embedded in Android and other mobile apps. Optimizing for size is only beginning however they think they can make CouchDB quite small and painless.