如图 说是absolute时如果没有设置left和top时 它不会占据位置 这里该怎么理解？
我尝试了一下如下的代码：
<style type="text/css"> .z{ position: relative; background-color: #00c0ef; margin: 10px 20px 10px 20px; } .p{ background-color: #00a65a; position: absolute; width: 200px; }</style><body> <div class=" z"> zpc <div class=" p"> zpc </div> </div></body> 然后发现实际效果是这样：
也就是说，当没有设置left和top时，内嵌的div并没有被包括在外部的div中 也就是不占位置，但是这个究竟是什么原理？
回复讨论(解决方案)
这时.z的relative写不写都是一样的，因为.p已经不以父元素为参考元素:
However; if an absolute positioned element has no positioned ancestors, it uses the document body, and moves along with page scrolling.
这时.z的relative写不写都是一样的，因为.p已经不以父元素为参考元素:
However; if an absolute positioned element has no positioned ancestors, it uses the document body, and moves along with page scrolling.
.p不是没有设定TRBL吗 应该以父级为参照点 就是.z?
这时.z的relative写不写都是一样的，因为.p已经不以父元素为参考元素:
However; if an absolute positioned element has no positioned ancestors, it uses the document body, and moves along with page scrolling.
还是说我理解错了 如果没有定义TRBL 则直接以body为参照？
如果这样 那div.p怎么会有这样的偏移量呢向左偏了和z一样的10px?
这时.z的relative写不写都是一样的，因为.p已经不以父元素为参考元素:
However; if an absolute positioned element has no positioned ancestors, it uses the document body, and moves along with page scrolling.
还是说我理解错了 如果没有定义TRBL 则直接以body为参照？
如果这样 那div.p怎么会有这样的偏移量呢向左偏了和z一样的10px?
看了半天终于懂了= =
绝对定位position:absolute;说它不占位置，是因为它不再影响后面兄弟元素的定位。
你可以这样理解：未设置定位的元素是二维的，它们之间在同一平面，互相影响，而设置了定位的元素是三维的；
绝对定位相当于它跑到另外一个平面去了，当然对以前的平面不再有影响；
另外，当然 相对定位也可以这样理解，只不过它比较霸道，不但自己跑到另外一个平面去了，原来的平面上的位置还要给它留着。
不知道你看懂了没？如果懂了，恭喜你！那么你肯定也懂了 z-index属性在什么情况下会起作用了
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