为什么没有发生外边距叠加呢？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
a[title]{
border-bottom:1px dotted #999;
}
a[title]:hover, a[title]:focus{
cursor:help;
}
.box1{ background:#234;
border:1px solid
#000;
height:50px;
width:50px;
margin:30px;
}
.box2{ background:#F00;
height:50px;
width:50px;
margin:20px;}
</style>
</head>
<body>
<p>The term <a href="#" title="self-contained underwater breathing apparatus">SCUBA </a> is an aconym rather than an abbreviation as it is as a word.</p>
<div class="box1"><div class="box2"></div></div>
</body>
</html>
图像7893回答
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qq_青枣工作室_0
因为你设置了border。W3C原文：Two margins are adjoining if and only if:both belong to in-flow block-level boxes that participate in the same block formatting contextno 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两个外边距仅在以下情况下叠加：都属于常规文档流(in-flow)中的 block-level boxes，而且处于同一个块级格式化上下文没有 line boxs 、clearance、padding 和 border 将其分隔(certain zero-height line boxes 除外)都属于垂直方向上相邻的盒子的相邻外边距 -
柠檬酸钠
因为div2在div1里面，而margin是外边距，这没法叠加啊 -
老猿
你把box2的margin去掉就重叠了！
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