Mosaic is a set of services, libraries together with a specification that defines how its components interact with each other, to support a microservice style architecture for large scale websites. While decomposing the backend into microservices is a widely adopted approach to achieve flexibility in development and operation, most frontend solutions are still running as a monolithic application. Mosaic addresses this issue by using Fragments that are served by separate services and are composed together at runtime according to template definitions. As a result, Fragments can be iterated on very rapidly, be more flexible in technology choices, and can better benefit from the extreme development pace of today's frontend technologies.