:empty

div:empty {
  outline: 2px solid deeppink;
  height: 1em;
}

<p>Element with no content:</p>
<div></div>

<p>Element with comment:</p>
<div><!-- A comment --></div>

<p>Element with nested empty element:</p>
<div><p></p></div>

:empty {
  /* ... */
}

<div class="box"><!-- I will be lime. --></div>
<div class="box">I will be pink.</div>
<div class="box">
  <!-- I will be pink in older browsers because of the whitespace around this comment. -->
</div>
<div class="box">
  <p>
    <!-- I will be pink in all browsers because of the non-collapsible whitespace and elements around this comment. -->
  </p>
</div>

body {
  display: flex;
  justify-content: space-around;
}

.box {
  background: pink;
  height: 80px;
  width: 80px;
}

.box:empty {
  background: lime;
}