What do pre-built authentication UIs look like?

Authentication is a critical aspect of user experience in applications, with several providers offering pre-built UI solutions that vary in customization capabilities. AWS Cognito's hosted UI, integrated with its OAuth server, offers limited customization, with constraints such as a fixed font and language support only in English. Auth0's Universal Login provides more flexibility through a no-code dashboard, customization pages, and a Management API, but significant customization can be time-consuming despite detailed documentation. Firebase Authentication is focused on mobile and offers minimal customization, prompting users to build custom UI components utilizing Firebase SDKs. Keycloak, an open-source provider, offers the ability to create custom themes through its documentation, though the process is often challenging and requires substantial effort. SuperTokens, also open-source, supports various authentication strategies and allows UI customization through CSS edits and React component overrides, offering a balance between ease of use and control over authentication flow design.