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<div class="box">Ovo je box</div>

.box {
  position: relative;
  width: 200px;
  height: 300px;
  border-left: 20px solid #999;
  border-bottom: 20px solid #ccc;
}
.box:before {
  content: " ";
  position: absolute;
  top: 0;
  left: -20px;
  z-index: 10;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 20px 20px 0 0;
  border-color: #fff transparent transparent transparent;
}
.box:after {
  content: " ";
  position: absolute;
  top: 100%;
  left: 100%;
  margin-left: -20px;
  z-index: 10;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 0 0 20px 20px;
  border-color: transparent transparent #fff transparent;
}

// ----
// Sass (v3.4.5)
// Compass (v1.0.1)
// ----

.box {
  //outline: 1px solid;
  position: relative;
  width: 200px;
  height: 300px;
  
  border-left: 20px solid #999;
  border-bottom: 20px solid #ccc;
  
  &:before {
    content: " ";
    position: absolute;
    top: 0;
    left: -20px;
    z-index: 10;
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 20px 20px 0 0;
    border-color: #fff transparent transparent transparent;
  }
  
  &:after {
    content: " ";
    position: absolute;
    top: 100%;
    left: 100%;
    margin-left: -20px;
    z-index: 10;
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 0 0 20px 20px;
    border-color: transparent transparent #fff transparent;
  }
  
}

<div class="box">Ovo je box</div>