line-height css

/* Keyword value */
line-height: normal;

/* Unitless values: use this number multiplied
by the element's font size */
line-height: 3.5;

/* <length> values */
line-height: 3em;

/* <percentage> values */
line-height: 34%;

/* Global values */
line-height: inherit;
line-height: initial;
line-height: unset;
.green {
  line-height: 1.1;
  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;
}
<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 <h1> line-height results from the red div's font-size  (15px × 1.1) = 16.5px,  probably not what you want -->