Develop a user story incrementally using a technique called vertical slicing. Drive a thin vertical slice through the story, from UI to DB, which is functionally coherent and demonstrable then progressively widen with consecutive slices.
When coding a login screen, simply show the username box and a button and allow a login with just the username typed (no checking). Then add a password box and still allow the login with no checking. Then have the user enter a username that is checked. Then a password.
Be sure to get feedback after every slice is completed. Ask a tester to perform some exploratory testing. Get a web designer to eyeball the UI. Demonstrate the emerging functionality to the Product Owner. From his perspective, it's like watching a picket fence being erected. After a few slats are installed a picture will form in his mind of how the end product will look and based on that the feedback provided will give you a steer going into the next slice.
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