樣式設置高度撐bai滿整個頁面，那麼必須從最外層，一層一層設置高度100%；
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
<style>
* {
padding: 0;
margin: 0;
box-sizing: border-box;
}
html,
body,
div {
height: 100%;
}
.app {
background: #ccc;
}
</style>
</head>
<body>
<div class="app"></div>
</body>
</html>