Edit in JSFiddle

<div class="wrapper">
    <div class="inner">
        innerだよ
    </div>
    <div class="spacer">
    </div>
</div>

.wrapper {
    width: 200px;
    background: red;
    position: relative;
}
.spacer {
    height: 200px;
}
.inner {
    display: table;
    width: 100%;
    height: 50%;
    position: absolute;
    top: 0;
    left: 0;
    background: yellow;
}