A sort-of similar but different approach to @Pekka’s: use the :after pseudo-selector, like so:
.mybox {
position: relative;
padding: 10px 20px;
background-color: #EEEEEE;
}
.mybox:after {
content: '';
position: absolute;
bottom: 0px;
left: 25%;
width: 50%;
border-bottom: 1px solid #0000CC;
}<div class="mybox">
Le content de box.
</div>…and a jsFiddle for good measure.