a {
    background: #A9A9A9;
    display: table;
    position: absolute;
    right: 0;
    top: 0;
    height: 100%;
    width: 20%;
    vertical-align: middle;
    float: right;
    text-align: center;
}

a span {
    display: table-cell;
    vertical-align: middle;
}