<div class="box">  
</div>
.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;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.