html结构：
<html>
<body>
<header></header>
<main>
<section></section>
</main>
<footer></footer>
</body>
</html>
scss：
body {
display: flex;
flex-direction: column;
min-height: 100vh;
header {
position: sticky;
top: 0;
width: 100%;
z-index: 999;
}
main {
flex: 1;
width: 100%;
}
}