line-height

/* 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: revert;
line-height: revert-layer;
line-height: unset;

The line-height property is specified as any one of the following:

• a <number>
• a <length>
• a <percentage>
• the keyword normal.

normal

Depends on the user agent. Desktop browsers (including Firefox) use a default value of roughly 1.2, depending on the element's font-family.

<number> (unitless)

The used value is this unitless <number> multiplied by the element's own font size. The computed value is the same as the specified <number>. In most cases, this is the preferred way to set line-height and avoid unexpected results due to inheritance.

<length>

The specified <length> is used in the calculation of the line box height. Values given in em units may produce unexpected results (see example below).

<percentage>

Relative to the font size of the element itself. The computed value is this <percentage> multiplied by the element's computed font size. Percentage values may produce unexpected results (see the second example below).

Use a minimum value of 1.5 for line-height for main paragraph content. This will help people experiencing low vision conditions, as well as people with cognitive concerns such as Dyslexia. If the page is zoomed to increase the text size, using a unitless value ensures that the line height will scale proportionately.

W3C Understanding WCAG 2.1

line-height = 
  normal                     |
  <number [0,∞]>             |
  <length-percentage [0,∞]>  

<length-percentage> = 
  <length>      |
  <percentage>  

/* All rules below have the same resultant line height */

/* number/unitless */
div {
  line-height: 1.2;
  font-size: 10pt;
}

/* length */
div {
  line-height: 1.2em;
  font-size: 10pt;
}

/* percentage */
div {
  line-height: 120%;
  font-size: 10pt;
}

/* font shorthand */
div {
  font:
    10pt/1.2 Georgia,
    "Bitstream Charter",
    serif;
}

It is often more convenient to set line-height by using the font shorthand as shown above, but this requires the font-family property to be specified as well.

This example shows why it is better to use <number> values instead of <length> values. We will use two <div> elements. The first, with the green border, uses a unitless line-height value. The second, with the red border, uses a line-height value defined in ems.

HTML

<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 -->

CSS

.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;
}

Result

• font, font-size