As an interdisciplinary group of developers, researchers, and designers, we are currently exploring collaborative co-located applications that invite users to engage with interactive audiovisual media using the mobile devices they carry with them. This work simultaneously investigates basic concepts of collaborative interactions and user experience, together with technical concepts and features. Allowing for rapid prototyping, easy deployment, and spontaneous participation, these explorations are entirely based on established and upcoming web standards such as WebSockets, WebGL, and Web Audio API. After an overview of basic concepts common to these applications, this paper describes Collective Loops, a prototypical interactive audiovisual installation that instantiates these concepts and allows for discussing important details in the design and implementation of applications of its kind. The final part of the article summarizes the incremental design and validation of two versions of the Collective Loops installation presented at public events.