Form with Javascript Funtions & Responsive CSS

What are you most proud of, and what would you do differently next time?

I'm proud of the function in Javascript. I would do it next time with ES5.

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

The form validation didn't work, because there were two not compatible elements. It took some time till I found it.

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

Responsive CSS. I don't get it.