内层div的margin-top影响外层div——引出外边距合并Collapsing margins

内层div的margin-top影响外层div——引出外边距合并Collapsing margins

作者：zccst

今天才算是了解边距合并。正如一位前辈所言，每一个CSS的坑，都让你学到不少知识。

<style type="text/css">
body { margin:0; padding:0;}
div { margin:0; padding:0;}
.a { width:500px; margin:0px auto 30px; background:#eee;}
.d1 { height:100px; width:500px; margin-top:30px; margin-bottom:30px; background-color:#f40;}
.d2 { height:100px; width:500px; margin-top:30px; margin-bottom:30px; background-color:blue;}

</style>

<div class="a">
    <div class="d1"></div>
    <div class="d2"></div>
</div>

<style type="text/css">
.b { /*height:200px; background:#ddd;*/ margin-top:50px; margin-bottom:50px;}

</style>
<div class="b"></div>

/*
出现原因是符合垂直边距合并的条件。
(No.6)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.
顶部发生合并，也就是本文的原因。注意发生条件是：外部div没有border，没有padding，而且与第一个子div之间没有任何间隔

消除办法：

一、字面直观解决办法
1，给外面div加padding-top:1px可以解决。
对于margin-top分三种情况：
（1）两个margin-top都是正数。margin-top:1px不行，原因是合并取其大。
（2）一正一负。如margin-top:-30px;两者互相抵消。离顶部距离等于30-(40/30/20)，如果是-40之类的话，顶部一部分被隐藏了
（3）两个都是负值。
2，给外面div加border:1px solied transparent;

另外，如果在内层div的上面有内容的话，也是正常的。由于这是不可能的，所以就没有单列一条。

二、非字面解决办法

3，为父元素添加float:left可以。但是仅设置子元素float:left不行。
(No.1)Margins between a floated box and any other box do not collapse (not even between a float and its in-flow children).

[4]，为父元素添加overflow：hidden；样式即可（完美）。overflow可以的原因是生成了新的块元素。
(No.2)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.

5，为父元素添加position:abosulte可以，relative不行。子元素设置position任何值都不行。
(No.3)Margins of absolutely positioned boxes do not collapse (not even with their in-flow children).

6，为子元素加display:inline-block也可以。
(No.4)Margins of inline-block boxes do not collapse (not even with their in-flow children).

疑问：float:left和position:absolute时上部有很大的空隙

底部也会发生类似的合并。
(No.7)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.

-------------------- 两个兄弟元素之间的margin合并情况 -----------------------------
(No.5)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.（两个<div></div>其它内容<div></div>）

-------------------- 一个内部元素自身与in-flow子孙的margin合并的情况 -----------------------------
(No.8)
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.
结论：与测试事实不符。

In this specification, the expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin. 所有毗邻的两个或更多盒元素的margin将会合并为一个margin共享之。毗邻的定义为：同级或者嵌套的盒元素，并且它们之间没有非空内容、Padding或Border分隔。
简单讲：根据规范，一个盒子如果没有上补白和上边框，那么它的上边距应该和其文档流中的第一个孩子元素的上边距重叠

*/

官方文章：

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, except:

• Margins of the root element's box do not collapse.
• If the top and bottom margins of an element with clearance are adjoining, its margins collapse with the adjoining margins of following siblings but that resulting margin does not collapse with the bottom margin of the parent block.

Horizontal margins never 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 (Note that certain zero-height line boxes (see 9.4.2) are ignored for this purpose.)
• 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
  • bottom margin of box and top margin of its next in-flow following sibling
  • bottom margin of a last in-flow child and bottom margin of its parent if the parent has 'auto' computed height
  • top and bottom margins of a box that does not establish a new block formatting context and that has zero computed 'min-height', zero or 'auto' computed 'height', and no in-flow children

A collapsed margin is considered adjoining to another margin if any of its component margins is adjoining to that margin.

Note. Adjoining margins can be generated by elements that are not related as siblings or ancestors.

When two or more margins collapse, the resulting margin width is the maximum of the collapsing margins' widths. In the case of negative margins, the maximum of the absolute values of the negative adjoining margins is deducted from the maximum of the positive adjoining margins. If there are no positive margins, the maximum of the absolute values of the adjoining margins is deducted from zero.

If the top and bottom margins of a box are adjoining, then it is possible for margins to collapse through it. In this case, the position of the element depends on its relationship with the other elements whose margins are being collapsed.

• If the element's margins are collapsed with its parent's top margin, the top border edge of the box is defined to be the same as the parent's.
• Otherwise, either the element's parent is not taking part in the margin collapsing, or only the parent's bottom margin is involved. The position of the element's top border edge is the same as it would have been if the element had a non-zero bottom border.

Note that the positions of elements that have been collapsed through have no effect on the positions of the other elements with whose margins they are being collapsed; the top border edge position is only required for laying out descendants of these elements.