Here is a simple card UI inspired login form from Sean Mann that has a nice transition between the Sign In & Register UI. When you click on register the form slides away to reveal the Name and Email input field with the sign me up button and similarly when you click on sign in the username, password field and the enter button slides in.
If you are having trouble with the pen, try the archived copy on GitHub
Sign In to Register CSS Snippet
See the Pen Login Form ( Only CSS ) by Sean Codes (@sean_codes) on CodePen.0