Vertical Margin Collapsing:

If two block-level elements are next to each other (e.g., one above the other), their vertical margins might "collapse." Horizontal margins do not collapse.

👶 Vertical-Align : impacts how inline or inline-block elements align relative to each other.

Grid 🆚 Flexbox

parent decides 🆚 child decides their width

Modal

position fixed된 애들을 겹겹이 쌓는게 아니라, 하나만 쌓고 그걸 flexbox로 만들어서 그 안에 element들을 포함시키면 된다.