The icon that is supposed to be displayed on the webauthn button (when fixed to be visible see #30936 ) is a decorative image so should not have an alt description. Or at most an alt description that describes the image.

The current alt description for the image is Login without a password using the W3C Web Authentication (WebAuthn) standard in compatible browsers. You need to have already set up WebAuthn authentication in your user profile.

This is obviously not a description of the image and anyone using a screen reader has to listen to this very long message before they can proceed.