WebAuthn is introducing the concept of Conditional Mediation (PR, explainer). This lets websites delegate offering available WebAuthn credentials to the browser depending on credential availability. The credentials are displayed alongside passwords on an autofill prompt. To select which input fields display webauthn credentials, they are marked autocomplete="webauthn" by the website. We should add this capability to the HTML spec.

We want to let websites express that an input should be filled by webauthn credentials and passwords or usernames. So webauthn should be an optional autofill detail token that can co-exist with other tokens, e.g.:

<label for="name">Username:</label>
<input type="text" name="username" autocomplete="username webauthn">
<label for="password">Password:</label>
<input type="password" name="password" autocomplete="password webauthn">