Much of this is specific to how I plan on implementing Authl.
- When a URL is entered into the form, an AJAX request will ask the Authl implementation if it's a supported profile. If so, a "Great!" message will appear, otherwise it'll say something like "I don't recognize that, try something else"
- The list of providers would be generated automatically by what's supported by the Authl configuration
- URLs in the provider list can be clicked on, and will fill out the input with the appropriate template and the cursor insertion point where the username goes
- The "supported providers" and "try one of these" lists would be generatable from data provided by the Authl instance in a structured manner.