The WebAuthn API can register a resident key on the YubiKey. This will basically store the username, private key and domain on the YubiKey. The website then can

The WebAuthn API can register a resident key on the YubiKey. This will basically store the username, private key and domain on the YubiKey. The website then can later request authentication based off a resident key. This will cause your web browser to query the YubiKey for resident keys of the website. You then can select the resident key with the correct username and will be logged in based on strong cryptography without needing to enter your password or username. Depending on your YubiKey configuration you might need to enter your YubiKey pin for this to work. See the screenshot in this comment on a GitHub issue: https://github.com/keepassxreboot/keepassxc/issues/3560#issuecomment-610773697 https://github.com/keepassxreboot/keepassxc/issues/3560#issu... The website will need to support this of course. Also the amount of storage available for resident keys on the YubiKey is limited.