Special Behavior of Margins

In this lesson, we will explore some special kinds of behavior of the margin property. Let's begin!

CSS has a feature called collapsing margins where two separate margins become a single margin.

The Listing below shows two situations when this phenomenon occurs.

Listing: Spacing with percentages

<!DOCTYPE html>
<html>
<head>
  <title>Spacing with percentages</title>
  <style>
    body {
      font-family: Verdana, Arial, sans-serif;
    }

    h1 { margin-bottom: 25px; }
    p { margin-top: 20px; }

    #warning {
      border: 2px dotted dimgray;
      padding: 8px;
    }
    h2 { margin: 16px; }
    #head {
      background-color: navy;
      color: white;
    }
    #head p { margin: 16px; }
  </style>
</head>
<body>
  <h1>Heading with margin-bottom: 25</h1>
  <p>Paragraph with margin-top: 20</p>
  <div id="warning">
    <h2>Did you know...</h2>
    <div id="head">
      <p>It is special!</p>
    </div>
    <p>
      Lorem ipsum dolor sit amet, consectetur
      adipiscing elit fusce vel sapien elit
      in malesuada semper mi, id sollicitudin.
    </p>
  </div>
</body>
</html>

According to the style sheet, the <h1> tag has a 25-pixel bottom margin, and the adjacent <p> tag has a 20-pixel top margin. These two are 45-pixels altogether. However, as ...