Digital Twins have been primarily developed and used to virtualize individual standalone physical assets, resulting in vertical application stacks. Going beyond this perspective, recent proposals envision ecosystems of connected digital twins that virtualize complex realities composed of a multitude of heterogeneous but interrelated physical assets — possibly belonging to different domains. This vision puts forth a number of engineering challenges, at the center of which is the capability of integrating digital twins developed with heterogeneous technologies. To this end, in this paper, we propose and discuss an approach based on Web standards and technologies for creating interoperable ecosystems of digital twins that applications can access and exploit as a service while being agnostic to the underlying heterogeneous technologies.