Here is a CSS only payment UI designed by Stas Melnikov, it shows you the front and back of a credit card and allow users to input the card details and cvv. If you want to test it out with some dummy credit card numbers you can use this fake credit card number generator.
If you are having trouble with the pen, try the archived copy on GitHub
Credit Card Snippet
See the Pen CSS Payment Card by Stas Melnikov (@melnik909) on CodePen.0