Why Does `margin-top: -8px` Shift Up While `margin-bottom: 8px` Shifts Down?

How Negative CSS Margins Work and the Distinction Between Top and Bottom Margins

In CSS, negative margins are used to create visual effects by altering the position of elements. When applied to the top margin, negative values create an upward shift, while negative bottom margins push elements downward.

Consider the example:

.item {
    position:absolute;
    top:50%;
    margin-top:-8px; /* half of height */
    height: 16px;
}

Here, the negative margin-top value of -8px shifts the element upward by half its height. This is achieved by extending the margin box (invisible spacing surrounding the element) above the edge of the content box (visible area).

Why margin-top:-8px ≠ margin-bottom:8px?

Although seemingly similar, margin-top:-8px and margin-bottom:8px differ in their effects. While the former shifts the element upward, the latter pushes it downward. This difference arises from the location of the anchor point for margins.

In CSS, all margins are anchored at the bounding box edge of the containing block. For a positioned element, the containing block is the element's parent. When using margin-top, the anchor point is at the top edge of the parent's bounding box. Thus, negative values move the element above this point.

Conversely, margin-bottom anchors the element at the bottom edge of the containing block. As a result, negative bottom margins move the element downward instead of pushing it upward.

Intuitive Understanding

Negative top margins effectively reduce the height of the margin box, creating the illusion that the element has moved upward. Negative bottom margins, on the other hand, increase the height of the margin box, making it appear as if the element has shifted downward.

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