Six div elements. Each has margin: 1em 0. Margins are collapsed.
We want no collapsing and also want to keep the exact sizes so using padding or border is a no go.
Use inline-block in 100% width. Leave the root element alone to avoid double margins.
Inline blocks are probably the most solid way to do this (least disadvantages).
Use floats in 100% width. Can't as easily avoid double margins. (There is extra space after this paragraph.)
Floats require much more CSS if you want to achieve the same result as with inline blocks:
But it's only clearfix that is needed, which you probably already have in your project.
Use overflow: hidden. Disadvantage is that you can't position stuff outside their parent. Dirty, but easy single line solution.