/*
引用该样式表，需在head标签内添加以下代码
    <link rel="stylesheet" href="./css/toTopBtn.css" type="text/css" /><!--复用返回顶部按钮样式-->
引用该样式表，需在body标签内添加以下代码
    <!--复用返回顶部按钮：开始-->
    <div id="toTopBtn" style="position: fixed;right: 10em;bottom: 10em;display: none;z-index: 1;" onclick="javascript:mainDIV.scrollTop=0;"><span><i class="toTopBtn"></i></span>
        <script>
            var mainDIV=document.getElementById("mainDIV");var toTopBtn=document.getElementById("toTopBtn");
            function showToTopBtn(){if(mainDIV.clientHeight+mainDIV.scrollTop==mainDIV.scrollHeight){toTopBtn.style.display="inline-block";};if(mainDIV.scrollTop==0){toTopBtn.style.display="none";};}
            window.addEventListener("scroll",showToTopBtn,true);
        </script>
    </div>
    <!--复用返回顶部按钮：结束-->
*/
.toTopBtn {
    display: inline-block;
    position: relative;
    vertical-align: middle;
    color: #ddd;
    font-size: 20px;
    font-style: normal;
    text-align: left;
    text-indent: -9999px;
    direction: ltr; 
    -webkit-box-shadow: 0 0 0 2px;
    box-shadow: 0 0 0 2px;
    border-radius: 4px;
    width: 1.5em;
    height: 1.5em;
    margin: 0; 
    -webkit-transform: rotate(-90deg);
    -ms-transform: rotate(-90deg);
    transform: rotate(-90deg); 
}
.toTopBtn:before, .toTopBtn:after {
    width: 0.707em;
    height: 2px;
    position: absolute;
    top: 0;
    bottom: 0;
    margin: auto 0;
    right: 0;
    -webkit-box-shadow: inset 0 0 0 1em;
    box-shadow: inset 0 0 0 1em;
    -webkit-transform-origin: right;
    -ms-transform-origin: right;
    transform-origin: right; 
    -webkit-transform: rotate(45deg);
    -ms-transform: rotate(45deg);
    transform: rotate(45deg); 
    content: '';
    pointer-events: none; 
}
.toTopBtn:before {
    width: 0.55em;
    right: 0.5em; 
}
.toTopBtn:after {
    -webkit-transform: rotate(-45deg);
    -ms-transform: rotate(-45deg);
    transform: rotate(-45deg); 
    width: 0.55em;
    right: 0.5em; 
}