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javascript - Warum kann margin:auto auto nicht nach oben, unten, links und rechts ausgerichtet werden?

過(guò)去多啦不再A夢(mèng) 2017-06-24 09:42:38

1270

Es gibt nur ein p in einem HTML.

<p class="test">
</p>

css1:

.test{
    width:200px;
    height:200px;
    border:1px solid red;
    margin:0 auto;
    }

css1 kann p.test links und rechts zentrieren.

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(qǐng)不要回答如何使他p.test上下左右居中，請(qǐng)回答為何margin:auto auto;不能上下左右居中？？

過(guò)去多啦不再A夢(mèng)

Antworte allen(4)

大家講道理2017-06-24 09:44:38 4 Etage

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?yàn)橐?guī)范就是這么規(guī)定的算法，瀏覽器就是這么實(shí)現(xiàn)的。