w3c

The general meaning of “margin” isn’t to convey “move this over by 10px” but rather, “there must be 10px of empty space beside this element.” I’ve always found this is easiest to conceptualize with paragraphs. If you just gave paragraphs margin-top: 10px and had no margins on any other elements, a series of paragraphs would … Read more

Transferring my comment to an answer, because it makes logical sense. However, please note that this is unfounded conjecture. The actual reasoning of why the spec is written this way is still, technically, unknown. Element height is defined by the height of the children. If an element has padding-top: 10% (relative to parent height), that … Read more