Mastering margin collapsing - CSS | MDN

<p>The bottom margin of this paragraph is collapsed …</p>
<p>
  … with the top margin of this paragraph, yielding a margin of
  <code>1.2rem</code> in between.
</p>

<div>
  This parent element contains two paragraphs!
  <p>
    This paragraph has a <code>.4rem</code> margin between it and the text
    above.
  </p>
  <p>
    My bottom margin collapses with my parent, yielding a bottom margin of
    <code>2rem</code>.
  </p>
</div>

<p>I am <code>2rem</code> below the element above.</p>

div {
  margin: 2rem 0;
  background: lavender;
}

p {
  margin: 0.4rem 0 1.2rem 0;
  background: yellow;
}