Mortgage repayment calculator - HTML, CSS and Javascript

What challenges did you encounter, and how did you overcome them?

I had huge problems getting customer CSS on the input fields. I did find one solution wih CSS :focus-within but it is not probably the best practice.

What specific areas of your project would you like help with?

I did not find yet a solution how to get the area next to the input field to turn red when there is missing information in inputs. I found a way to change the border to red but not the other area which has the €, % and year signs. If there is a easy solution for this please let me know :)