A CodePen by Dimitrios Zorbas.

index.html

<div class="popover">
  <div class="popover-arrow"></div>
</div>

style.css

.popover {
  position: relative;
  width: 200px;
	height: 100px;
	background-color: rgb(233, 212, 222);
	border-radius: 4px;
  top: 50px;
  left: 50px;
}


.popover-arrow {
	position: absolute;
	top: -10px;
	left: -10px;
	border-left: 20px solid transparent;
	border-right: 20px solid transparent;
	border-bottom: 20px solid blue;
  width: 0;
  height: 0;
  z-index: 100;
}