block-in-inline-negative-mb-collapses-with-positive-mt-ref.html

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<!DOCTYPE html>

<title>CSS Reference</title>

<style>

.container { width: 100px; }

.first { width: 100px; height: 20px; margin-bottom: -20px; background: blue; }

.second { width: 100px; height: 20px; margin-top: 50px; background: green; }

</style>

<p>Blue rectangle, 30px gap (50 - 20), green rectangle.</p>

<div class="container">

    <div class="first"></div>

    <div class="second"></div>

</div>