console
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<link rel="stylesheet" href="2.css">
</head>
<body>
第一种方法：<br>
<div class="box1">宽度固定</div>
<div class="box2">宽度不固定</div>
第二种方法：<br>
<div class="box3">宽度不固定</div>
<div class="box4">宽度固定</div>
</body>
</html>
.box1 {
width: 400px;
height: 200px;
background-color: tomato;
float: right;
}
.box2 {
width: 100%;
height: 200px;
background-color: yellow;
}
.box3{
position: absolute;
left:0;
right: 400px;
height: 200px;
background-color: rgb(117, 65, 238);
}
.box4{
position: absolute;
width:400px;
right: 0;
height: 200px;
background-color: green;
}