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  1. The line-height CSS property sets the height of a line box in horizontal writing modes
  2. In vertical writing modes, it sets the width of a line box
  3. It's commonly used to set the distance between lines of text
  4. On block-level elements in horizontal writing modes, it specifies the preferred height of line boxes within the element, and on non-replaced inline elements, it specifies the height that is used to calculate line box height

Values

  1. normal
    1. Depends on the user agent
    2. Desktop browsers (including Firefox) use a default value of roughly 1.2, depending on the element's font-family
  2. <number> (unitless)
    1. The used value is this unitless <number> multiplied by the element's own font size
    2. The computed value is the same as the specified <number>
    3. In most cases, this is the preferred way to set line-heightand avoid unexpected results due to inheritance
  3. <length>
    1. The specified <length> is used in the calculation of the line box height
    2. Values given in em units may produce unexpected results
  4. <percentage>
    1. Relative to the font size of the element itself
    2. The computed value is this <percentage> multiplied by the element's computed font size
    3. Percentage values may produce unexpected results

Example

Normal

Friends, Romans, countrymen, lend me your ears
I come to bury Caesar, not to praise him
The evil that men do lives after them
The good is oft interred with their bones

line-height

Friends, Romans, countrymen, lend me your ears
I come to bury Caesar, not to praise him
The evil that men do lives after them
The good is oft interred with their bones
CSS 
.lineheight {
    line-height: 2.5;
    border: 1px solid gray;
    padding: 1em;
}

References

MDN: line-height