Found a shorter version, again with linebreaks for readability:

    <p id=r>
    <script>WebAssembly.instantiate(Uint8Array.fromBase64(
    'AGFzbQEAAAABBwFgAn9/AX8DAgEABwUBAWEAAAoJAQcAIAAgAWoL'))
    .then(x=>r.append(x.instance.exports.a(51,4)))
    </script>