<div class="square">
    <p class="triangle"></p>
</div>

.square {
  position: relative;
  width: 400px;
  height: 200px;
  background: green;
  margin: 150px auto;
}
.triangle {
	position: absolute;
  top: -40px;
  left: 50%;
  margin-left: -20px;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 20px;
  border-color: transparent transparent red transparent;
  font-size: 0;
  line-height: 0;
}