Newsletter sign up form using basic HTML, CSS and Javascript

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

I'm unsure if I did this challenge correctly, I created the specification of the sign up form and success message as closely as I could, including the shadow and gradient on the buttons when hovered. I had the browser switch images from mobile to desktop when appropriate in regards to screen size. I also created a JavaScript file that checks to see if the submitted information is a valid email or not, if so it will then hide the form and display the success message and if not it restyles the form box red and displays an error message. I also had the site revert back to the sign up form when the dismiss message button is clicked.

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

Once again, any advice on how I can better structure my code, pointing out any radiancies or simpler ways I could achieve the same goal would be very much appreciated. Pointing out any mistake I have made regarding the challenge would also be helpful. Thank you.