css浮动回到顶部按钮

<div style="height:2000px;">
<p id='back-ltop' class='hide_element'>
    <a href='#top'><span></span</a>
</p>

<style>
    /*******Go to top ******/
  #back-ltop {
    position: fixed;
    z-index: 999;
    right: 35px;
    bottom: 165px;
  }

  /* arrow icon (span tag) */
  #back-ltop span {
    width: 40px;
    height: 40px;
    display: block;
    margin-bottom: 7px;
    background: #ddd url(/images/bs_revamp/download-icon.png) no-repeat center center;

    /* rounded corners */
    -webkit-border-radius: 15px;
    -moz-border-radius: 15px;
    border-radius: 15px;

    /* transition */
    -webkit-transition: 1s;
    -moz-transition: 1s;
    transition: 1s;
  }
</style>

<script>
  $(document).ready(function(){

    // hide #back-top first
    $("#back-ltop").hide();

    // fade in #back-top
    $(function () {
      $(window).scroll(function () {
        if ($(this).scrollTop() > 100) {
          $('#back-ltop').fadeIn();
        } else {
          $('#back-ltop').fadeOut();
        }

      });

      // scroll body to 0px on click
      $('#back-ltop a').click(function () {
        $('body,html').animate({
          scrollTop: 0
        }, 800);
        return false;
      });
    });
  });
</script>

</div>