html,css讓一個div位于頁面正中間

#canvasBox {
    width: 343px;
    height: 225px;
    background: #ffffff;
    border-radius: 3px;
    border: 1px solid #979797;
    position: absolute;
    top: 50%;
    left: 50%;
    -webkit-transform: translate(-50%, -50%);
    -moz-transform: translate(-50%, -50%);
    -ms-transform: translate(-50%, -50%);
    -o-transform: translate(-50%, -50%);
    transform: translate(-50%, -50%);
}