How can user authentication be set up on a Webflow site without using Firebase UI and instead building custom forms for full control over the site's appearance and functionality?

Setting up user authentication on a Webflow site without using Firebase UI and opting for custom forms can provide you with greater control over the appearance and functionality of your site. To achieve this, you can follow these steps:

1. Choose an Authentication Service: Since you won't be using Firebase UI, you need to select an alternative authentication service. One popular option is Auth0, which provides robust authentication capabilities and allows you to customize the authentication flow to match your site's design.

2. Set Up Auth0 Account and Configuration: Create an account on the Auth0 website and set up a new application for your Webflow site. In the Auth0 dashboard, configure the necessary settings, such as allowed callback URLs and allowed logout URLs, which should match your Webflow site's domain.

3. Design Custom Login and Signup Forms: In Webflow, design your own custom login and signup forms using form elements, input fields, and buttons. Customize the forms to match your site's design and layout.

4. Handle Form Submissions: Once a user submits the login or signup form, you'll need to handle the form data and communicate with the Auth0 API for authentication. You can accomplish this using either client-side JavaScript or server-side code, depending on your preference and security requirements.

5. Use Auth0 SDKs: Auth0 provides SDKs for different platforms and languages that you can use to interact with their API. These SDKs simplify the authentication process and provide helper functions for actions like logging in, signing up, and handling tokens and user information.

6. Authenticate Users: On the server or client-side, authenticate users by making API calls to Auth0 using the submitted form data. Auth0 will return a JWT (JSON Web Token) upon successful authentication, which you can store securely for subsequent authentication checks and authorization.

7. Implement Access Controls: With user authentication in place, you can now implement granular access controls and authorization logic based on user roles and permissions. You can manage this within your Webflow site's backend or server-side code, where you can check the validity and permissions of the JWT for each authenticated request.

8. Customize User Experience: Leverage Auth0's customization options to match the authentication flow with your Webflow site's design. You can customize the login page, error messages, and branding elements using the available configuration options in Auth0's dashboard.

9. Implement Forgot Password and Email Verification: Implement additional functionality like password reset and email verification using Auth0's APIs and hooks. You can create custom flows for these actions and integrate them with your Webflow site's user interface.

Remember to conduct thorough testing and security reviews to ensure your custom authentication implementation is secure and free from vulnerabilities. Additionally, staying up to date with the authentication service's documentation and best practices will help you make the most of the chosen solution.