Mobile first responsive page using HTML, CSS and vanilla JavaScript

This is my third challenge and I'm pretty happy with it. The first challenge was simple enough that I didn't really have to worry much about responsiveness. For the second challenge, I didn't use a mobile first approach and after getting the layout looking good on large screens, it fell apart on mobile size(s), and I had to start over again. This time around I used a mobile first approach and it was soooo much easier. Quite happy with the way this turned out, but of course, always open to suggestions for improvements, tips, etc. As for the javascript, I did have to look up how to verify an email address and found some crazy code that I can't imagine myself (or too many other people) coming up with. If there are better ways to do this, please let me know. Thanks!!