margin collapse

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碰到margin无效问题，查的一些资料。

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Margin</title>
<style type="text/css">
	.container{
		margin: 0 auto;
		width: 800px;
		padding: 0 10px 10px;
		outline: 1px solid red;
	}
	.item{
		height: 50px;
		margin-top: 10px;
		outline: 1px solid green;
	}
</style>
</head>
<body>
	<div class="container">
		<div class="item">默认margin</div>
		<div class="item"></div>
	</div>
</body>
</html>

实际结果：

第一个div的margin-top没生效,一般情况下是两个相邻的div的margin-bottom与margin-top会出现坍塌，而这里是父子两个元素。

期望结果：

IE浏览器是由所谓的hasLayout引起：

1.该盒子加浮动；
2.用内边距来解决（给其父元素一个设置个内边距）
3.overflow：hidden；
4.给其父元素一个设置个border属性也可解决。
5.绝对定位

参考：

http://adamghost.com/2008/12/ie-haslayout-%E8%AF%A6%E8%A7%A3/

http://www.cnblogs.com/imhurley/archive/2011/11/23/2260764.html

对于非IE浏览器可查看下面的W3C规范：

Here are the relevant points from the W3C spec:

8.3.1 Collapsing margins

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.

Adjoining vertical margins collapse [...]

Two margins are adjoining if and only if:

both belong to in-flow block-level boxes that participate in the same block formatting context

no line boxes, no clearance, no padding and no border separate them

both belong to vertically-adjacent box edges, i.e. form one of the following pairs:

top margin of a box and top margin of its first in-flow child

The reason why doing any of the following prevents the margin collapse:

Floating either of your div elements
Making either of of your div elements inline blocks
Setting overflow of #outer to auto (or any value other than visible)

Is because:

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

The left and right margins behave as you expect because: