Css 外边距折叠(collapsed margin ) 浅析_html/css

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Css 外边距折叠(collapsed margin ) 浅析_html/css_WEB-ITnose

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Jun 24, 2016 am 11:43 AM

Css 外边距折叠(collapses margin )

a.先来看看w3c 文档对于外边距折叠的定义：

In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.

翻译一下：在Css中，相邻盒子(有可能是同辈元素，也可能不是)的外边距可以合并成为一个外边距。外边距的这种合并方式称为折叠，这种合并过的外边距称为 外边距折叠。

举个例子：

<style type="text/css">.testBFC{    width:100px;    height:100px;    background-color:green;}.testBFC div{    width:30px;    height:30px;    background-color:pink;    }.testBFC div:first-child{    margin-bottom:10px;}.testBFC div:last-child{    margin-top:10px;}</style><div class="testBFC">    <div>div1</div>    <div>div2</div></div>

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运行结果如下图。从图中我们可以看出来，div1 的margin-bottom和 div2 的margin-top 合并了，结果 div1和div2 之间只有10px。我们本来是要他们之间间隔20px，如何实现呢？可以给div1 的class中添加：display：inline-block。这就涉及到如何解决或避免外边距折叠的问题了。

下面来看看w3c文档的具体注意事项:

Note the above rules imply that:

Margins between a floated box and any other box do not collapse (not even between a float and its in-flow children).

Margins of elements that establish new block formatting contexts (such as floats and elements with 'overflow' other than 'visible') do not collapse with their in-flow children.

Margins of absolutely positioned boxes do not collapse (not even with their in-flow children).

Margins of inline-block boxes do not collapse (not even with their in-flow children).

The bottom margin of an in-flow block-level element always collapses with the top margin of its next in-flow block-level sibling, unless that sibling has clearance.

The top margin of an in-flow block element collapses with its first in-flow block-level child's top margin if the element has no top border, no top padding, and the child has no clearance.

The bottom margin of an in-flow block box with a 'height' of 'auto' and a 'min-height' of zero collapses with its last in-flow block-level child's bottom margin if the box has no bottom padding and no bottom border and the child's bottom margin does not collapse with a top margin that has clearance.

A box's own margins collapse if the 'min-height' property is zero, and it has neither top or bottom borders nor top or bottom padding, and it has a 'height' of either 0 or 'auto', and it does not contain a line box, and all of its in-flow children's margins (if any) collapse.

1. 浮动的盒子和其它盒子之间不会发生折叠。(甚至浮动的盒子和它的正常流中子元素之间也不会发生折叠). 举个例子，下面例子中父元素和子元素之间会发生外边缘折叠，这时可以父元素或子元素设置成浮动，就可以显示正常。

<head>    <style type="text/css">        *{            margin:0;            padding:0;        }        .testBFC{            width:100px;            height:100px;            background-color:green;            margin-top:10px;        }        .testBFC div{            width:30px;            height:30px;            background-color:pink;        }        .testBFC div:first-child{            margin-top:10px;        }    </style></head><body><div class="testBFC">    <div>div1</div></div></body>

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2. 为元素的外边距建立一个新的 block formatting contexts(块级格式上下文),这样就不会和它的正常流中的子元素发生折叠。如上面的例子中，可以在外面的div 添加一个属性overflow:hidden

3. 绝对定位的盒子不会发生折叠。上面的例子中，可以给父元素或者子元素添加属性：position:absolute 或者 fixed.

4. 在一个正常的文档流中，块级元素的底部外边距回和它的后面的兄弟元素的上边距发生折叠，除非你设置了 clearance。见文章顶部的例子。

5. 块级元素的 top margin 会和它的第一个块级元素的子元素的 top margin 发生外边距折叠，如果这个块级元素没有设置 top border, no top padding ,子元素没有设clearance. 举例：tip 1 中已经有了这个例子。

6. 块级元素的bottom margin 有可能会和它的最后一个块级子元素的 bottom margin 发生外边距折叠，如果这个块级元素没有设置 bottom padding，bottom border 并且 height:auto, min-height:0

7. 这样的盒子会外边距折叠， min-height:0, 并且没有设置 top or bottom borders 或者 top or bottom padding，并且height:0 or auto 并且它不包含 line box 并且它所有的子元素外边距折叠。

详细信息请参考w3c文档: http://www.w3.org/TR/CSS2/box.html#collapsing-margins

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