Described is a technology by which code, such as an untrusted web application hosted in a browser, provides content through an interface for playback by an application environment, such as an application environment running in a browser plug-in. Content may be in the form of elementary video, audio and/or script streams. The content is in a container that is unpackaged by the application code, whereby the content may be packaged in any format that the application understands, and/or or come from any source from which the application can download the container. An application environment component such as a platform-level media element receives information from an application that informs the application environment that the application is to provide media stream data for playback. The application environment requests media stream data (e.g., samples) from the application, receives them as processed by the application, and provides the requested media stream data for playback.