box-shadow はさまざまな角度からの投影を実装します

width: 100px;
height: 100px;
background: #f1f1f1;
margin: 20px ;
box-shadow: #000 0 -5px 5px -5px;

width: 100px;
height: 100px;
background: #f1f1f1;
margin: 20px ;
box-shadow: #000 5px 0 5px -5px;

width: 100px;
height: 100px;
background: #f1f1f1;
margin: 20px ;
box-shadow: #000 0 5px 5px -5px;

width: 100px;
height: 100px;
background: #f1f1f1;
margin: 200px ;
box-shadow: #000 -5px 0 5px -5px;

width: 100px;
height: 100px;
background: #f1f1f1;
margin: 20px ;
box-shadow: 5px 0 5px -5px #000,-5px 0 5px -5px #000;