<!doctype html>
<!DOCTYPE html>
<html>
<head>
  <title></title>
</head>
<body>
  <h3>The example below shows a background with a background video and that has size 100%.</h3>
</body>
</html>
body {
  background-image: url('https://media0.giphy.com/media/OK5LK5zLFfdm/giphy.gif');
  background-clip: border-box;
  background-size: 100%;
}

h3 {
  color: white;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.