A method for discovering and installing web application plugins is disclosed. In one embodiment, such a method includes initiating a client-side component of a server-based web application. The client side-component hosts a first document. The method opens an inline frame, such as an IFrame or Web Worker, within the first document that hosts a second document. The method uses cross-document messaging to communicate, from the second document to the first document, metadata describing a plugin's capabilities and metadata describing an API to access the plugin's capabilities via cross-document messaging. The method utilizes the capabilities of the plugin using cross-document messaging in accordance with the API. A corresponding system and computer program product are also disclosed.