Prefer unitless numbers for line-height values

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/line-height -->

<div class="box blue">
 <h1>Avoid unexpected results by using unitless line-height</h1>
  length and percentage line-heights have poor inheritance behavior ...
</div>

<div class="box green">
 <h1>Avoid unexpected results by using unitless line-height</h1>
  length and percentage line-heights have poor inheritance behavior ...
</div>

<div class="box red">
 <h1>Avoid unexpected results by using unitless line-height</h1>
  length and percentage line-heights have poor inheritance behavior ...
</div>

<!-- The first <h1> line-height is calculated from its own font-size   (30px × 1.1) = 33px  --> 
<!-- The second and third <h1> line-height results from the parent div's font-size  (15px × 1.1) = 16.5px,  probably not what you want -->

CSS

.blue {
  line-height: 1.1;
  border: solid aqua;
}
.green {
  line-height: 110%;
  border: solid limegreen;
}
.red {
  line-height: 1.1em;
  border: solid red;
}
h1 {
  font-size: 30px;
}
.box {
  width: 18em;
  display: inline-block;
  vertical-align: top;
  font-size: 15px;
  padding: 1em 2em;
}