Как сделать это в css?

.elm { 
  position: relative; 
  width: 100px; 
  height: 100px; 
  border: 1px solid #ccc; 
} 
 
    .elm:before { 
      position: absolute; 
      content: ''; 
      left: 10px; 
      top: -1px; 
      width: 0; 
      height: 0; 
      border-style: solid; 
      border-width: 10px 10px 0 10px; 
      border-color: #ccc transparent transparent transparent; 
    } 
 
    .elm:after { 
      position: absolute; 
      content: ''; 
      left: 11px; 
      top: -1px; 
      width: 0; 
      height: 0; 
      border-style: solid; 
      border-width: 9px 9px 0 9px; 
      border-color: #fff transparent transparent transparent; 
    }

<div class="elm"></div>