<div></div>

div{
    display: block;
  position: relative;
  border: solid 20px #78A110;
border-left-color: rgba(0, 0, 0, 0);
border-right-color: rgba(0, 0, 0, 0);
border-bottom-width: 0;
  margin: 50px auto;
  width: 0;
}
div:after{
    content: '';
    display: inline-block;
    position: absolute;
    top: -20px;
    left: -20px;
    border: solid transparent 10px;
    border-top-width: 10px;
    border-top-color: black;
    border-left-width: 30px;
    opacity: .2;
}