如果相对放置的容器内的唯一元素的位置是绝对的，则必须在容器上指定高度。问题在于 position：absolue 从内容流中删除了元素，因此容器无法识别绝对定位的元素的高度。

If the only element inside your relatively positioned container is positioned absolute, you will have to specify a height on the container. The problem is that position:absolue removes the element from the content flow, so the container doesn't recognize the absolutely positioned element's height.