margin merge
https://developer.mozilla.org.

personal opinion: the height of the div box (if the declaration height is not displayed) is determined by the real content of the internal child elements, while the margin is not counted in the real content. You can view the console, and the real content is within the solid line

-supplement-

you can try to increase the property, within the h4 solid line, and you can find that the height of the div will change accordingly