how to set line spacing in html code example

Example 1: line spacing css

line-height: 20px; /* 4px +12px + 4px */
/* OR */
line-height: 1.7em; /* 1em = 12px in this case. 20/12 == 1.666666  */

Example 2: line-height css

<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 -->