WebAssembly (Wasm) is a popular portable assembly-like language. Besides browser support in the four most common browsers (Chrome, Firefox, Safari, Edge) a number of standalone Wasm engines are available. With several such independent implementations naturally follows a risk of disagreement between the individual implementations. To help ensure agreement between Wasm implementations, we develop a stack-directed program generator to drive differential testing of the four browsers’ Wasm engines. We describe our experimental setup, our development of a stack-directed shrinker for reducing a generated counterexample program, and finally report on a number of disagreements and bugs found. Surprisingly our black-box generator found 2 crashing bugs, despite browser vendor efforts to fuzz test their Wasm engines using a state-of-the-art fuzzer.