The WebAssembly spec already supports 128-bit fixed-width SIMD opcodes, see the spec and the clang builtins for details.

I'm wondering if the simdutf maintainers are interested in accepting a port to WebAssembly SIMD128. This won't be worked on immediately (for Safari it's only in TP and not release yet), but having a tracking ticket is good anyway, and I'm curious to hear what you folks think :)