Flexbox landing page + form validation with CSS only

This is a challenge that I've started about a month ago but i dropped it because I had no idea how to do the form validation using JS but yesterday I saw @ApplePieGiraffe's solution using only CSS then I decided to give it a try and I think the result looks pretty cool and I'm proud and happy I was able to make it! (By the time I was doing this challenge I wasn't using Sass or BEM, like I use in my newer projects, so it can be kinda messy)