Collapsing margin

The expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin. In CSS 2.1, horizontal margins never collapse.

Anything that stops adjoining margins from "touching" each other will prevent them from collapsing.

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aqua

yellow

pink

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<div id="aqua"><p>aqua</p></div> <div id="yellow"><p>yellow</p></div> <div id="pink"><p>pink</p></div>

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