Newsletter with Email Validation

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

I've upped my skills when inserting html thru javascript but along the way it gave me some difficulties and I see now that and html file is there for a reason. Now I would like to know when we really need to use HTML thru javascript, anybody?

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

The form validation was a challenge within 10 different little challenges. I love the code communities on w3school and github who helped me overcome the problems I was facing! I think it's working perfectly , what you do guys think?

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

My javascript code is far from optimal, If anybody has any tips or tricks they would be very helpful!