Fylo landing page, fully responsive using flexbox, vanilla html/css

I've been fiddling with this project for a while, refactoring it several times, cramming as much learning into it as possible. Started desktop-first, converted to mobile-first, and later added some breakpoints to make it responsive.

I've used px values. Thought about redoing it but decided to keep it for now and then learn about relative units in my next challenge. Preprocessors and frameworks are also subjects for a future challenge.

** My first challenge, any and all feedback is appreciated **

• I've redone the class-names many times, but they still feel a bit clunky. BEM-inspired. Are they understandable? Can they be optimized?
• HTML semantics, am I using the right tags? Can it be simplified?
• css file structure, does it make sense?
• Is the way I handle media queries somewhat meaningful? (There are a lot!)

Or anything else you can think of, from the minor details to the major blunders.