back top pure javascript

#返回顶部-原生javascript ##HTML 可以放在页面body中的任何位置##CSS a.back-top{ tdisplay: none; tposition: fixed; tright: 2em; tbottom: 4em; tt theight: 50px; twidth: 50px; tt tbackground-color: #da6426; tbackground-image: url(../images/scrollup.png); tbackground-position: center; tborder-radius: 7px; t-webkit-transition: 0.5s; t -moz-transition: 0.5s; t -ms-transition: 0.5s;