.box {

  /* Fallback: */

  background-image: url("http://satyr.io/300x300/");

  background-image: image-set( 

    "http://satyr.io/300x300/" 1x,

    "http://satyr.io/600x600/" 2x,

    "http://satyr.io/900x900/" 3x

  );

} 

​

/* ETC: */

​

.box {

  width: 300px;

  height: 300px;

  border: 3px solid darkgrey;

  background-size: cover;

}

​

body {

  margin: 1rem;

}