核心记忆点：
- 左右盒子margin负值，中间父盒子宽度100%，中间子盒子自适应
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
.left {
width: 200px;
background: red;
height: 500px;
float: left;
margin-left: -100%;
}
.right {
width: 300px;
background: blue;
height: 500px;
float: left;
margin-left: -300px;
}
.content {
float: left;
width: 100%;
}
.box {
background: green;
height: 500px;
margin-left: 200px;
margin-right: 300px;
}
</style>
</head>
<body>
<div class="wrap">
<div class="content">
<div class="box"></div>
</div>
<div class="left"></div>
<div class="right"></div>
</div>
</body>
</html>
