:empty

/* Selects any <div> that contains no content */
div:empty {
  background: lime;
}

:empty

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

.box {
  background: red;
  height: 100px;
  width: 100px;
}

.box:empty {
  background: lime;
}

<div class="box"><!-- I will be lime --></div>
<div class="box">I will be red</div>
<div class="box">
    <!-- I will be red because of the whitespace around this comment -->
</div>