Our research examines the use of CodeCircle, an online, collaborative HTML, CSS, and JavaScript editor, as a rapid prototyping environment for musically expressive instruments. In CodeCircle, we use two primary libraries: MaxiLib and RapidLib. MaxiLib is a synthesis and sample processing library, ported from the C++ library Maximillian, which interfaces with the Web Audio API for sound generation in the browser. RapidLib is a product of the Rapid-Mix project, and allows users to implement interactive machine learning, using "programming by demonstration" to design new expressive interactions.