Credit card form with pure CSS, HTML, JS

Hello, this is the first challenge I completed here. I would be happy about some feedback!

The most difficult part of creating the website was the validation, because on my first try, the special input system for the expire date (2 inputs) did not work out, because there should be only one message for both inputs. I created a own class for the input validation and managed to implement that behaviour. I did not manage to show the light shade next to the card, because it would then also be displayed on the background image. Does someone know how to do it?

What do you think about my solution, how can I improve?