Form validation using Javascript

For me, the most difficult part was the form validation using Javascript (I did not use the HTML form validation API as the pop-ups which are displayed are really ugly and I did not manage to modify them...) If anyone knows an alternative for this it would be much appreciated

Also, I used javascript to add 1 space between each 4 numbers in the card number. After reviewing many solutions I took the one which I considered as the best one, but I see that it is not always working (especially if you type very fast) Any idea here would also be much welcomed

Thanks !