Maybe you’re right and you could implement something like MLS in JS and/or WASM. I don’t know if those can open raw socket connections instead of using websockets. You’d need a raw socket connection to interoperate with other apps like Signal, but with websockets you could implement similar functionality non-interoperably, maybe even using a server side proxy to route to a Signal server (the proxy would never see plaintext).

There’s still the matter of making sure that the JS or WASM download isn’t tampered with. You have to trust the server that send it to you, which is in conflict with the E2EE notion of not trusting the server. There could possibly be a MITM attack by someone with a signed TLS certificate. Worst is if it’s served from a CDN. CDN’s have alway seemed to me like supply chain attacks waiting to happen.