Why doesn't the div element stretch to its full height?

Question

HTML

Why doesn't the div stretch to the full height of div.main?
https://codepen.io/valek-web/pen/WNwYeOJ

Answer 1

height:100% for this div, remove
100% will not work, because main also has 100%, and its parent has no height. This means that main does not have a height, which means that the div does not either.
If you use flexes - use flexes, no need to use crutches.