a,button {
    width: 150px;
    height: 150px;
    border: 1px solid #000;
    text-align: center;
}

a {
    display: table;
    -moz-box-sizing: border-box;
}

a span, button span {
    vertical-align: middle;
    text-align: center;
}

a span {
    display: table-cell; 
}