Position your <div> absolutely at the bottom and don't forget to give div.A a position: relative - http://jsfiddle.net/TTaMx/
.A {
position: relative;
margin: 40px 0;
height: 40px;
width: 200px;
background: #eee;
}
.A:after {
content: " ";
display: block;
background: #c00;
height: 29px;
width: 100%;
position: absolute;
bottom: -29px;
}?