They're kinda unrelated. I don't think WebAssembly has explicit SIMD operations yet? Then again, maybe WebAssembly could be delivered in a form which makes autovectorization straightforward for some subset of use cases.
There's a proposal for SIMD but it's not in any shipping browsers AFAIK.
We already have an enscripten'd decoder, built into a nice analyzer tool that lets you inspect the AV1 bitstream! But you're right, it'll need SIMD to decode at high resolutions in realtime.