CindyGL: authoring GPU-based interactive mathematical content. CindyJS is a framework for creating interactive (mathematical) content for the web. The plugin CindyGL extends this framework and leverages WebGL for parallelized computations.{par} CindyGL provides access to the GPU fragment shader for CindyJS. Among other tasks, the plugin CindyGL is used for real-time colorplots.{par} We introduce the main principles, concepts and application of CindyGL and describe the encountered technical challenges. Special focus is put on a novel visualization scheme that uses feedback loops, which were among the motivating forces of developing CindyGL. They can be used for a wide range of applications. Some of them are numerical simulations, cellular automatons and fractal generation, which are described here.