Implement passkey autofill (conditional UI) using mediation:'conditional' in navigator.credentials.get
domain: w3.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Add autocomplete='username webauthn' to the username input field so the browser can annotate the autofill dropdown with passkey suggestions.
Check for conditional mediation support: call PublicKeyCredential.isConditionalMediationAvailable() and only proceed if it returns true.
Call navigator.credentials.get({ publicKey: { challenge, rpId, userVerification }, mediation: 'conditional' }) on page load — this call does not show a modal; it resolves when the user selects a passkey from the autofill UI.
Abort the conditional get before showing a traditional password form submit or if the user manually triggers a full-modal passkey prompt; use an AbortController signal.
On resolution, perform the full server-side assertion verification — userHandle returned in the assertion identifies the user, eliminating the need for a username field.
Known gotchas
The conditional mediation call must be initiated before the user interacts with the autofill field; calling it after the field receives focus will miss the browser's autofill decoration window.
Only one active conditional get request should exist per page; a second call without aborting the first may be silently ignored or rejected by the browser.
The feature requires both browser support (Chrome 108+, Safari 16+, Firefox 119+) and that the authenticator has stored a discoverable credential for the rpId.
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