<div class="box">
  
</div>
.box {
  width: 0;
  height: 0;
  background: transparent;
  border-top: 10px solid transparent;
  border-left: 10px solid transparent;
  border-right: 10px solid #00BBFF;
  border-bottom: 10px solid #00BBFF;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.