1.上下/左右相邻的普通元素margin，不是两者相加之和，而是取最大值，这个现象叫做margin重叠。
2. 普通元素才会发生margin重叠，如果是float元素，就不会发生。margin是两者相加之和。
例如：1普通元素
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>练习(margin重叠现象）</title>
<style>
#test1{
height: 100px;
background: blue;
margin-bottom: 50px;
}
#test2{
height: 100px;
background: green;
margin-top:50px;
}
</style>
</head>
<body>
<div id="test1"></div> <div id="test2"></div>
</body>
</html>
2，浮动元素：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>练习(margin重叠现象）</title>
<style>
#test1{
height: 100px;
width: 200px;
background: blue;
margin-bottom: 50px;
float: left;
}
#test2{
height: 100px;
width: 200px;
background: green;
float: left;
clear: left;
margin-top:50px;
}
</style>
</head>
<body>
<div id="test1"></div>
<div id="test2"></div> </body>
</html>
3,父子（普通元素）发生margin重叠现象
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>练习(margin重叠现象）</title>
<style>
#test1{
height: 150px;
width: 200px;
background: blue; }
#test2{
height: 50px;
width: 50px;
background: green; margin-bottom:50px;
}
#test3{
height: 50px;
width: 50px;
background: green;
margin-top: 50px;
}
</style>
</head>
<body>
<div id="test1">
<div id="test2"></div>
<div id="test3"></div>
</div>
</body>
</html>
4，父子（子为浮动元素）不发生margin重叠
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>练习(margin重叠现象）</title>
<style>
#test1{
height: 150px;
width: 200px;
background: blue; }
#test2{
height: 50px;
width: 50px;
background: green;
float: left;
margin-bottom:50px;
}
#test3{
height: 50px;
width: 50px;
background: green;
float: left;
clear: left;
margin-top: 50px;
}
</style>
</head>
<body>
<div id="test1">
<div id="test2"></div>
<div id="test3"></div>
</div>
</body>
</html>
