Edit in JSFiddle

<div class="box">width:300px;
    <div class="inner">Width:100%;border-left:5px solid #red;</div>
</div>

.box {
    width: 295px;
    height: 100px;
    background: gray;
}
.inner {
    width: 100%;
    color: white;
    background: black;
    border-left: 5px solid red;
    box-sizing: border-box;
}