Here's one way: http://codepen.io/anon/pen/HKbrq
HTML:
<div class="trapezoid"> </div>
CSS:
.trapezoid {
width: 400px;
height: 150px;
border-style: solid;
border-width: 40px;
border-color: transparent;
border-left-color: black;
border-left-width: 300px;
}