CSS

.upper {
  position:absolute;
  left:150px; top:100px; /* pos.*/   }

  .upper div {
    border-left-color: transparent;
    border-style: solid;
  }
  .inner{
  border-width: 0 0 /* x: */ 100px /* h: */ 100px;
  border-bottom-color: #955;
}

HTML

<div class="upper">
<div class="inner"></div>
</div>

Example