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True - worth pointing out that this library gracefully falls back to ASM.js if your browser does not support WebAssembly, though.

Does it fallback if Window.WebAssembly doesn't exist, or does it fallback if WebAssembly.instantiateStreaming fails?

The former. In practice, it seems some runtimes are still a bit buggy - so we also revert to ASM.js if e.g. you are on an iPhone