arrowbox.css

#box{
  display: block;
  height: 100px;
  width: 100px;
  background: green;
  position: relative;
  opacity: .5;
  border: 2px solid blue;
}

#box:after{
  content: "▼";
  color: green;
  display: block;
  position: absolute;
  left: 78%;
  bottom: -12px;
  text-shadow: 0px 4px 0px blue;
  -webkit-transform: scaleY(.7);
}