There is only one p in an html.
<p class="test">
</p>
css1:
.test{
width:200px;
height:200px;
border:1px solid red;
margin:0 auto;
}
css1 can center p.test left and right.
css2:
.test{
width:200px;
height:200px;
border:1px solid red;
margin:auto auto;
}
為何css2不可以使p.test上下左右居中？？
請不要回答如何使他p.test上下左右居中，請回答為何margin:auto auto;不能上下左右居中？？ 
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The following constraints must hold among the used values of the other properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block
If both 'margin-left' and 'margin-right' are 'auto', their used values are equal. This horizontally centers the element with respect to the edges of the containing block.
下面是關(guān)于高度的：
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0. If 'height' is 'auto', the height depends on whether the element has any block-level children and whether it has padding or borders:
因為規(guī)范就是這么規(guī)定的算法，瀏覽器就是這么實現(xiàn)的。
CSS規(guī)范
This is a good question. Let me ask you, when there are two elements in a row, can you center them left and right? In the same way, when there may be elements above and below you, how do you want the browser to calculate the position where the corresponding element should exist?
No, if you don’t use flex layout for top and bottom alignment, there is no good way. Virtical–align is not easy to use either!
No, if you want to center the top, bottom, left, and right, it is recommended to use flex layout, followed by table, table-cell or positioning
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