In this paper, we present a generic framework for performance evaluation of video streams. We embrace the perspective of a streaming application, and model the playback buffer level at given network and playback conditions. That way, our model enables flexible, adaptable evaluation of a wide variety of streaming methods and protocols. We test our model on HTML5 and Flash video as served by the YouTube web site. We find universal achievable limits in any streaming process, and explore quality trade-offs every application has to decide on.