A user-agent-based application is a public client in which the client code (generally javascript) is downloaded from a web server and executes within a user-agent (e.g., web browser) on the device used by the resource owner (ie end user).
Protocol data and credentials are easily accessible (and often visible) to the resource owner (ie end user).
Since such applications reside within the user-agent, they can make seamless use of the user-agent capabilities when requesting authorization.