function initMoving(target) {
 if (!target)
  return false;

 var obj = target;
 obj.initTop = 278;
// obj.initLeft = 865;
 obj.bottomLimit = document.documentElement.scrollHeight - 220;
 obj.topLimit = 150;

 obj.style.position = "absolute";
 obj.top = obj.initTop;
// obj.left = obj.initLeft;
 obj.style.top = obj.top + "px";
// obj.style.left = obj.left + "px";

 obj.getTop = function() {
  if (document.documentElement.scrollTop) {
   return document.documentElement.scrollTop;
  } else if (window.pageYOffset) {
   return window.pageYOffset;
  } else {
   return 0;
  }
 }
 obj.getHeight = function() {
  if (self.innerHeight) {
   return self.innerHeight;
  } else if(document.documentElement.clientHeight) {
   return document.documentElement.clientHeight;
  } else {
   return 500;
  }
 }
 obj.move = setInterval(function() {
  pos = obj.getTop() + obj.getHeight() / 2 - 140;

  if (pos > obj.bottomLimit)
   pos = obj.bottomLimit
  if (pos < obj.topLimit)
   pos = obj.topLimit

  interval = obj.top - pos;
  obj.top = obj.top - interval / 3;

  obj.style.top = obj.top + "px";
 }, 40)
}

<div>
 <div><br><br></div>
 <div id="floater">浮动内容</div>
</div>