Couchbase Lite is a native library so you can’t use it in a web app that can run in a standard web browser. You can use it in a web-app packaged as an application via PhoneGap, though.
If you want something that will run in a regular browser and can sync, take a look at PouchDB, which is a pure-JS library that uses the HTML5 local database and the CouchDB sync protocol. The Couchbase Sync Gateway is compatible with this sync protocol so you can use PouchDB as a client for it.
—Jens