how to make a backgroud image to cover the whole screen in css? code example

Example 1: css background full screen

body { 
  background: url(img/bg-image.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

Example 2: how to cover whole webpage by background image in html

background: url(images/bg.jpg) no-repeat center center fixed; 
  background-size: cover;