Open Player Settings for WebGL > Publishing Settings > Linker Target
Ensure that Linker Target is set to Both. This will produce a WebAssembly build in addition to the fallback asm.js format for browsers that do not support WebAssembly. (see the Unity User Manual for more information).
Note that Unity 2019 no longer supports the asm.js fallback, so there is no longer an option to set the Linker Target.