CSS line-height Tutorial

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1- CSS line-height

CSS line-height defines a distance between 2 baselines in 2 consecutive lines of text of an element. Baseline is the bottom line of most letters on the same line. Take a look at the following illustration:

Line-height vs font-size

It demonstrates the relationship between line-height and font-size.
/* Keyword value (Default) */
line-height: normal;

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

/* <length> values */
line-height: 3em;
line-height: 20px;

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

/* Global values */
line-height: inherit;
line-height: initial;
line-height: unset;
If an element has an unspecified  line-height, it will inherit from the parent element. Possible values of CSS line-height are:

normal

is the default value of the  line-height. This value is dependent on the browser. Basically, it is 20% bigger than the  font-size. For example, if the font-size is 14px, the line-height will be around  16.8px.

«number» (unitless)

If you provide a «number» value excluding the unit for the line-height, the value of the line-height will be equal to the number multiplied by the font-size. This approach is recommended, which helps you avoid some unwanted results from inheritance.

«length»

Provide a value with a specific unit, for example 30px, 25pt, etc.
Note: Using an EM unit with a CSS line-height may lead to an unexpected result. You can see it in the example below.

«percentage»

Specify a percentage value compared to the font-size of the current element.
Note: Using a percentage unit with a CSS line-height may also produce an unexpected result. See it in the example below.

Example:
lie-height-example.html
<!DOCTYPE html>
<html>
<head>
    <title>CSS line-height</title>
    <meta charset="UTF-8"/>

     <style>
       .option {
           padding: 5px;
           display:inline-block;
           border: 1px solid gray;
       }
       #my-div {
          background-color: #ddd;
          margin-top: 5px;
       }
    </style>

    <script>
        function changeLineHeight(value)  {
           var e = document.getElementById("my-div");
           e.style.lineHeight= value;
        }
    </script>
</head>
<body>
    <h3>CSS line-height</h3>

    <div class="option">
       <p style="color:red;">Line-height:</p>
       <input name ="x" type="radio" onClick="changeLineHeight('normal')" checked/> normal <br/>
       <input name ="x" type="radio" onClick="changeLineHeight('3em')" /> 3em <br/>
       <input name ="x" type="radio" onClick="changeLineHeight('3.5')" /> 3.5 <br/>
       <input name ="x" type="radio" onClick="changeLineHeight('35px')" /> 35px <br/>

       <input name ="x" type="radio" onClick="changeLineHeight('150%')" /> 150%
    </div>

    <div id="my-div">
        Line 1 <br/>
        Line 2 <br/>
        Line 3 <br/>
        Line 4
    </div>

</body>
</html>

2- Line-height and EM problem

When using CSS line-height with the  EM unit, you may get an unexpected result. The reason is that CSS line-height is inherited. But the inheritance is different between  line-height with unit and unitless line-height. Take a look at the example below:
The <H1> elements in this example have inherited line-height from their parent element, which may be smaller than the font-size. This makes the text look awful on display.
unexpected-line-height-em.css
.green {
   line-height: 1.1;
   border: solid limegreen;
 }

 .red {
   line-height: 1.1em;
   border: solid red;
 }

 h1 {
   font-size: 30px;
 }

 .box {
     width: 160px;
     display: inline-block;
     vertical-align: top;
     font-size: 15px;
 }
unexpected-line-height-em.html
<!DOCTYPE html>
<html>
<head>
    <title>CSS line-height & EM</title>
    <meta charset="UTF-8"/>
    <link rel="stylesheet" type="text/css" href="unexpected-line-height-em.css" />

</head>
<body>
    <h3>Unexpected result of CSS line-height and EM unit.</h3>

    <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>

</body>
</html>

3- Line-height and % problem

When using CSS line-height along with % unit, you may get an unexpected result. The reason is that CSS line-height is inherited. However, the inheritance is different between  line-height with unit and unitless line-height. Take a look at the example below:
The <H1> elements in this example have inherited line-height from their parent element, which may be smaller than the font-size. This makes the text look awful on display.
unexpected-line-height-percent.css
.green {
   line-height: 1.1;
   border: solid limegreen;
}

.red {
   line-height: 110%;
   border: solid red;
}

h1 {
   font-size: 30px;
}

.box {
     width: 160px;
     display: inline-block;
     vertical-align: top;
     font-size: 15px;
}
unexpected-line-height-percent.html
<!DOCTYPE html>
<html>
<head>
    <title>CSS line-height & %</title>
    <meta charset="UTF-8"/>
    <link rel="stylesheet" type="text/css" href="unexpected-line-height-percent.css" />

</head>
<body>
    <h3>Unexpected result of CSS line-height and %.</h3>

    <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>

</body>
</html>

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