Language Workbenches are instruments developers use to create new domain-specific languages. They provide tools to rapidly develop, test and deploy new languages. Currently, workbenches support deployment in desktop-based integrated development environments. Setting up these environments can be a hurdle for the often non-technical users of these languages. Web-Based IDEs could be a solution in this case, but workbenches are currently not able to deploy languages in these environments. This work presents the first step towards language workbenches in Web IDEs by creating a language parametric runtime for the browser which serves as a back-end for Spoofax. Combined with an editor, this runtime is the basis for the generation of entirely client-side language playgrounds based on Spoofax specifications. For parsing, this runtime has similar performance characteristics as the existing Spoofax implementation. Code execution in this runtime can be used in environments where performance is not critical.