If you implement this, it works with Android, too. Login via WebAuthN with Fingerprint works with Android since months. I tested it successful in december last year.
Hanashi
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This example works for me:
<input type="submit" id="MyButton">
<script>
		async function webauthnCheck() {
				if (await PublicKeyCredential.isUserVerifyingPlatformAuthenticatorAvailable()) {
						document.getElementById('MyButton').onclick = async function(event){
								event.preventDefault();
								const options = {
										publicKey: {
												rp: {
														name: 'mydomain.tld'
												},
												user: {
														id: Uint8Array.from("XXXXXX", c => c.charCodeAt(0)),
														name: 'test',
														displayName: 'Test User'
												},
												challenge: Uint8Array.from('XXXXXX', c => c.charCodeAt(0)),
												pubKeyCredParams: [
														{
																type: 'public-key',
																alg: -7
														}
												],
												timeout: 15000
										}
								}
								navigator.credentials.create(options).then(function (publicKeyCredential) {
										console.log(publicKeyCredential);
								})
						}
				}
		}
		webauthnCheck();
</script>
Yes, this works! I just tested this as I was curious myself, and when Safari asked me for Touch ID my watch would also give me the option to authorise a sign in. But it only works if the Mac Book is unfolded. If the Mac Book is folded and connected with an external display, the API for TouchID does not work.
The problem is known. I found this entry in the bugtracker for webkit: https://bugs.webkit.org/show_bug.cgi?id=213595
I have the same problem.
I enabled it, but it doesn't work, yet.
Edit: it only works, if your Mac Book is unfolded, because if it's folded, you can not use TouchID ;)