It's actually using webassembly if available, otherwise it fallbacks on asm.js >If true, the miner will always use the asm.js implementation of the hashing alg

It's actually using webassembly if available, otherwise it fallbacks on asm.js >If true, the miner will always use the asm.js implementation of the hashing algorithm. If false, the miner will use the faster WebAssembly version if supported and otherwise fall back to asm.js. The default is false. Source : https://coin-hive.com/documentation/miner https://coin-hive.com/documentation/miner