You could achieve this with layering some borders, but it's a little easier if you do it with transforms / background gradients. Here's a class that can travel well (I just set it up for webkit now, but you can mod easily).
.container {
width:450px;
height:150px;
background: grey;
border-radius: 2px;
}
.container:after {
display: block;
content:"";
width: 450px;
height: 150px;
position: absolute;
z-index: -1;
top: 150px;
background: linear-gradient(170deg, pink, transparent 40%);
-webkit-transform-origin: 0 0 0;
-webkit-transform: skewX(50deg) rotateX(75deg);
}
And in the fiddle: http://jsfiddle.net/jaaLx/