WebAssembly is a new low-level language currently being implemented in all major web browsers. It is designed to become the universal compilation target for the web, obsoleting existing solutions in this area, such as asm.js and Native Client. The WebAssembly working group has incorporated formal techniques into the development of the language, but their efforts so far have focussed on pen and paper formal specification.

We present a mechanised Isabelle specification for the WebAssembly language, together with a verified executable interpreter and type checker. Moreover, we present a fully mechanised proof of the soundness of the WebAssembly type system, and detail how our work on this proof has exposed several issues with the official WebAssembly specification, influencing its development. Finally, we give a brief account of our efforts in performing differential fuzzing of our interpreter against industry implementations.