background-image without cover code example

Example 1: is there an img cover

body {
  margin: 0;
}
img {
  display: block;
  width: 100vw;
  height: 100vh;
  object-fit: cover;
}

Example 2: how to set background automatically with my screen height

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