<div class="em">
  <div>
    In em div is 16.2<span>, span is 14.58px</span>
  </div>
</div>
<div class="rem">
  <div>
    In rem, div is 14.4<span>, span is 14.4</span>
  </div>
</div>

body { font-size: 20px;}

.em, .em div { font-size: .9em;}
.em span { font-size: .9em;}

.rem, .rem div { font-size: .9rem;}
.rem span { font-size: .9rem;}