Older browsers don't have the global WebAssembly object available to instantiate and load Wasm modules. There are experimental polyfills that utilize asm.js if the object isn't found, but the WebAssembly Working Group currently has no plans to create one. Since asm.js and WebAssembly are closely related, simply serving up an asm.js file if the WebAssembly object is unavailable will still offer performance gains while accommodating for backward compatibility. You can see which browsers currently support WebAssembly at https://caniuse.com/#feat=wasm.