<div id="rem">
  <span>A
    <span>B
      <span>C</span>
    </span>
  </span>
</div>

<div id="em">
  <span>A
    <span>B
      <span>C</span>
    </span>
  </span>
</div>
#rem span {
  font-size: 2rem;
  color: darkorchid;
}

#em span {
  font-size: 2em;
  color: crimson;
}

html {
  font-family: arial;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.