定位示例

#middleBox
{
    position:    absolute;
    top:         275px;
    left:        50%;    /* move the left edge to the center … */
    margin-left: -275px; /* … and move it to the left half the Box’ width. */
    z-index:     9999;   /* Try to get it on top. */
}

使用像Dragonfly或Firebug这样的工具来检查属性是否仍然消失.