回到顶部按钮

滚动条滚动到下一屏出现回到顶部按钮，点击回到顶部按钮滚动条回到顶部。

    
        
        
            #btn {
                width:40px;
                height:40px;
                position:fixed;
                left:50%; 
                display:none;
                margin-left:610px;
                bottom:30px; 
                background:url(http://img1.sycdn.imooc.com/535e0dc100010e9c00400080.jpg) no-repeat left top;
            }
            #btn:hover {    
                background:url(http://img1.sycdn.imooc.com/535e0dc100010e9c00400080.jpg) no-repeat 0 -39px;
            }
            .box {
                width:1190px; 
                margin:0 auto; 
            }
    
    
    
        
            
        
        
        
        
    

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