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效果图
<!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>
html,body,div,ul,li,h1,h2,h3,h4,h5,h6,p,dl,dt,dd,ol,form,input,textarea,th,td,select{margin:0;padding:0}
*{-webkit-box-sizing:border-box;-moz-box-sizing:border-box;box-sizing:border-box}
html,body{min-height:100%}
body{font-family:Helvetica,Pingfang SC,Microsoft YaHei,STHeiti,Verdana,Arial,Tahoma,sans-serif;font-size:14px;color:#333;background:#fff;position:relative}
h1,h2,h3,h4,h5,h6{font-weight:normal}
ul,ol{list-style:none}
img{border:none;vertical-align:middle}
a{color:#666;text-decoration:none}
a:visited{color:#666;text-decoration:none}
a:hover{color:#666;text-decoration:none}
a:active{color:#666;text-decoration:none}
table{border-collapse:collapse;table-layout:fixed}
input,textarea{outline:none;border:none}
textarea{resize:none;overflow:auto}
.clearfix{zoom:1}
.clearfix:after{content:".";0;height:0;visibility:hidden;display:block;clear:both;overflow:hidden}
/* 本案例CSS */
body .pageUp{background: #e4e4e4;position: fixed;height: 50%; 100%;top: -50%;z-index: 9999999;transform: translateY(100%);transition: all 1.2s ease-in-out 0.2s;border-bottom: 1px solid #ccc;}
body .pageDown{background: #e4e4e4;position: fixed;height: 50%; 100%;bottom: -50%;z-index: 9999999;transform: translateY(-100%);transition: all 1.2s ease-in-out 0.2s;border-top: 1px solid #ccc;}
body.active .pageUp{transform: translateY(0);}
body.active .pageDown{transform: translateY(0);}
</style>
</head>
<body>
<!-- 遮罩框 S-->
<div class="pageUp"></div>
<div class="pageDown"></div>
<!-- 遮罩框 E-->
<div>导航条</div>
<div>网站内容</div>
<div>底部</div>
<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<script>
$(function () {
setTimeout(function () {
$('body').addClass('active')
}, 0)
})
</script>
</body>
</html>