SOURCE

<div class='rectangle'>
  <div class="triangle"></div>
  <div class='demo'></div>
</div>

.rectangle {
  width: 200px;
  height: 120px;
  border: 2px solid #27ab3d ;
  border-radius: 5px;
  position: relative;
  left:200px;
}
 .triangle {
    position: absolute;
    top:-8px;
    left:-9px;
    border-right: 2px solid #27ab3d;
    border-top: 2px solid #27ab3d;
    height: 10px;
    width: 10px;
    margin:50px auto 0;
    -webkit-transform: rotate(228deg);
    border-left: 2px solid transparent;
    border-bottom: 2px solid transparent;
} 
.opacity {
  width: 5px;
  height:5px;
  background: red;
  position: absolute;
  left:-5px;
  top:20px
}
.demo {
  width: 0;
    height: 0;
    border-width: 10px;
    border-style: solid;
    border-color: #f3f5f6 transparent transparent;
    transform: rotate(90deg);
    position: absolute;
    left: -19px;
    top: 39px;
}