We revisit Alan Kay's early vision of dynamic media that blurs the distinction between documents and applications. We introduce shareable dynamic media that are malleable by users, who may appropriate them in idiosyncratic ways; shareable among users, who collaborate on multiple aspects of the media; and distributable across diverse devices and platforms. We present Webstrates, an environment for exploring shareable dynamic media. Webstrates augment web technology with real-time sharing. They turn web pages into substrates, i.e. software entities that act as applications or documents depending upon use. We illustrate Webstrates with two implemented case studies: users collaboratively author an article with functionally and visually different editors that they can personalize and extend at run-time; and they orchestrate its presentation and audience participation with multiple devices. We demonstrate the simplicity and generative power of Webstrates with three additional prototypes and evaluate it from a systems perspective.