當內容的高度足夠長的時候，footer應該會跟在main元素的後面
當main元素比較短的時候，這個footer元素能夠粘連在屏幕的底部
<!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;
}
html,body{
height: 100%;
}
#wrap{
/* 最小高度100% */
min-height:100%;
background-color: darkgray;
text-align: center;
}
.main{
padding-bottom: 50px;
}
.footer{
height:50px;
background-color: darkcyan;
text-align: center;
line-height: 50px;
margin-top: -50px;
}
</style>
</head>
<body>
<div id="wrap">
<div class="main">
main <br>
main <br>
main <br>
main <br>
main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br> main <br>
main <br>
main <br>
main <br>
</div>
</div>
<div class="footer">
footer
</div>
</body>
</html>