x
 
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
div {
  width: 100%;
  height: 400px;
  background-image: url('/i/photo/flower-5.jpg');
  background-repeat: no-repeat;
  background-size: contain;
  border: 1px solid red;
}
</style>
</head>
<body>
<p>请调整浏览器窗口大小来查看效果。</p>
<div></div>
</body>
</html>