Responsive single page using flexbox

I would like (and would be so grad) if someone see my solution and told me if I used tags unnecessary. Maybe sugest good names for my css classes.

I programation, I learned someday, that there is not good and better code for the solution. What there is, is your solution. "you solved the problem".

But in front-end, I would be so grad, if someone told to me, helping me to improve my self. Thanks!