<style>
.box{
    position:fixed;
    right:10px;
    bottom: 10px;
    height:30px;
    width: 50px;    
    text-align:center;
    padding-top:20px;    
    background-color: lightblue;
    border-radius: 20%;
    overflow: hidden;
}
.box:hover:before{
    top:50%
}
.box:hover .box-in{
    visibility: hidden;
}
.box:before{
    position: absolute;
    top: -50%;
    left: 50%;
    transform: translate(-50%,-50%);
    content:'回到顶部';
    width: 40px;
    color:peru;
    font-weight:bold;

}    
.box-in{
    visibility: visible;
    display:inline-block;
    height:20px;
    width: 20px;
    border: 3px solid black;
    border-color: white transparent transparent white;
    transform:rotate(45deg);
}
</style>
</head>
<body style="height:2000px;">
<div id="box" class="box">
    <div class="box-in"></div>
</div>   
 
