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<style>
#goTop{
display:none;
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bottom:20px;
background:url(http://cdn.w3cfuns.com/resource/images/scrolltop.png) no-repeat 0 0;
width:34px;
height:34px;
cursor:pointer;
right:20px;
}
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window.onload=function(){
var timer=null;
var goTop=document.getElementById("goTop");
goTop.onclick=function(){
timer=setInterval(function(){
var top=document.body.scrollTop+document.documentElement.scrollTop;
var speed=top/5;
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<body>
<p>123</p><br/>
<p>123</p><br/>
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<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
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