<div class="container">
<span class="inner"</span>
</div>
<div class="container border">
<span class="shadow"></span>
<span class="inner"></span>
</div>

.container {
  width: 300px;
  height: 100px;
  background: yellow;
  position: relative;
  float: left;
  margin-right:20px;
}
.border{
 border:2px solid #820010;
}
.inner,.shadow {
    display: block;
    float: left;
    width: calc(50% - 25px);
    height: 0;
    border-top: 50px solid grey;
    border-bottom: 50px solid grey;
    border-right: 50px solid transparent !important;
    border-left: none;
  position: absolute;
}
.shadow{
 width: calc(50% - 23px);
    position: absolute;
    border-color: #820010;
}
}