/* 頁面頂部居中顯示 */
.head {
    position: absolute; 
    top: 0;
    left: 0;
    right: 0;
    bottom: auto;
    margin: auto;
}
/* 頁面底部居中顯示 */
.foot {
    position: absolute; 
    top: auto;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;
}
/* 頁面上下左右居中顯示 */
.center {
    position: absolute; 
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;
}