


CSS margin collapse: Why does my margin-top move the parent element?
Apr 05, 2025 pm 02:54 PMDetailed explanation of CSS margin collapse: Cleverly solve margin problems
When learning CSS layout, margin
attributes often bring some unexpected results, especially the "margin collapse" phenomenon. This article will use a case to deeply analyze the causes and solutions of margin collapse.
Question: Assume in one<canvas></canvas>
Nested an element inside<div> element (id is "one"), and set <code>margin-top
for "one". Strangely, after setting margin-top
, not only the "one" element is moved downward,<canvas></canvas>
The element also moved downwards.
Code structure (simplified):
HTML: <canvas><div id="one">...</div></canvas>
CSS: #one { margin-top: 20px; }
(Other styles are omitted here)
Analysis: This is CSS margin collapse. When the parent element height is auto
, the child element is a block-level element, and the parent element has no padding
and border
, the height of the parent element will be affected by the child element margin-top
.<canvas></canvas>
The height of an element is calculated automatically, while "one" is a block-level element, its margin-top
and<canvas></canvas>
The top margins of<canvas></canvas>
The height of the body increases, thus moving downward as a whole.
Solution: There are several ways to avoid margin collapse:
- Set the parent element height:
<canvas></canvas>
The element specifies a clear height value, for exampleheight: 100px;
. - Add border or inner margin: Add
border
orpadding
to the parent element, such asborder: 1px solid #ccc;
orpadding: 10px;
. - Use inner margin instead of outer margin: Replace
margin-top
withpadding-top
. - Use floating or absolute positioning: set
float: left;
orposition: absolute;
for child element "one".
By understanding the mechanism of margin collapse, we can better control the CSS layout and avoid similar problems.
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