Responsive newsletter sign-up using CSS Flex and some JS functions

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

I am most proud of the way I did transitions on the page. They are smooth (almost) and it looks the way I wanted it to look. My issue is - as always - unreadable code. As I am coming up with different solutions to problems and sometimes those don't work, I have to edit my code several times and leave some mess after changes. I am trying to use the method of creating test files and experiment inside them, but it's hard for me to work with multiple files at once and remember to create new ones every time I edit something.

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

The greatest challenge in this project was to make my main block change it's size depending on the screen size using media query. It wouldn't be that hard if I didn't want to make my beloved transitions :) but since I wanted my page to look smooth (and the only "smooth" thing is main block size change) - I had to make some size rules for my main block and it was not easy to be honest.

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

The only help I need right now is to know - how to write readable css and html code. I lack some default setups for my files and basic ways things are done in general.