The dashed orange outline is a display: flow-root wrapper, so it's stretched by the real margins inside it. Solid blue is the container, light blue the children. A gap between orange and blue is margin reaching outside the container; orange sitting flush against blue means there's no margin there.

Positive margins, margin-trim: none

Positive margins, margin-trim: block

Negative margins, margin-trim: none

Negative margins, margin-trim: block