为什么我把窗口逐渐拉小后，左、中、右板块会分离？
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{
margin: 0;
padding: 0;
}
.header,.footer{
height: 50px;
line-height: 50px;
background-color: gray;
text-align: center;
}
.container{
padding: 0 220px 0 200px;
overflow: hidden;
}
.middle,.left,.right{
float: left;
height: 300px;
position: relative;
}
.middle{
width: 100%;
background-color: red;
}
.left{
width: 200px;
background-color: blue;
margin-left: -100%;
left: -200px;
}
.right{
width: 220px;
background-color: green;
margin-left: -220px;
right: -220px;
}
</style>
</head>
<body>
<div class="header">头部</div>
<div class="container">
<div class="middle">中</div>
<div class="left">左</div>
<div class="right">右</div>
</div>
<div class="footer">底部</div>
</body>
</html>
