绝对定位-随滚动条 固定于某处

css:　　

#anchor{   position:absolute;

   top:40%;   left:40%;   width:100px;   height:100px;   background-color:red;}

js:$(function(){   var anchorTop = $("#anchor").css("top");   anchorTop = Number(anchorTop.substring(0, anchorTop.indexOf("p"))); //首先在监听器外部记录某id=anchor的标签的初始位置   window.onscroll = function () {      var top = document.documentElement.scrollTop || document.body.scrollTop;      $("#anchor").css({         transition:"top 1s",         webkitTransition:"top 1s",         oTransition:"top 1s",         msTtransition:"top 1s",         mozTransition:"top 1s",         top: anchorTop + top + "px"      });   };});