Sign-up Form using VanillaJS and CSS (see readme for React alternate)

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

I'm proud that I was able to solve the challenge in two ways, one using vanillajs (submitted here) and another using React.

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

Figuring out how to get the exact styling I wanted for certain situations was tricky. Especially things like showing blank input box with valid state styling, while showing the button styling to be invalid. Getting the modal and overlay styled correctly also took a lot of googling. It seems the options on this must have been updated recently.

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

I want to learn how to simplify my code as much as possible. Also, comments on best practices are helpful!