margin重叠现象

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>