SOURCE

<div class="BoxWrap">
     <div class="horn">
        <div class="lt"></div>
        <div class="rt"></div>
        <div class="rb"></div>
        <div class="lb"></div>
     </div>
  </div>

.BoxWrap{
        width: 100px;
        height: 100px;
        position: relative;
      }
      .horn{
        position: absolute;
        width: 100%;
        height: 100%;
        border:1px solid #00d3e7;
      }
      .horn>div{
        width: 10px;
        height: 10px;
        position:absolute;
      }
      .horn .lt{
        border-top: 3px solid #00d3e7;
        border-left: 3px solid #00d3e7;
        left: -2px;
        top: -2px;
      }
      .horn .rt{
        border-top: 3px solid #00d3e7;
        border-right: 3px solid #00d3e7;
        right: -2px;
        top: -2px;
      }
      .horn .rb{
        border-bottom:3px solid #00d3e7;
        border-right: 3px solid #00d3e7;
        right: -2px;
        bottom: -2px;
      }
      .horn .lb{
        border-bottom:3px solid #00d3e7;
        border-left: 3px solid #00d3e7;
        left: -2px;
        bottom: -2px;
      }