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JavaScript 
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<title>はじっこにいすわる3</title>

<style type="text/css"> 
#a { height:3000px; }

</style>

<div id="a"></div>
<div id="menu1">
<a href="#top">Top</a></div>
<script type="text/javascript">
//@cc_on

function Decorator (alpha) {
  //@ this.filter = 'Alpha(opacity=' + alpha + ')';
  this.opacity  = alpha / 100 + '';
}

function Locator (x, y) {
  this.left = x + 'px';
  this.top  = y + 'px';
}

function Starter (callbackfn) {
  this.timerID = (function (o) {
    return setInterval(function () { return callbackfn.call(o); }, o.interval);
  })(this);
}

//___

Elevator = function (eId, m_sec) {
  var e = document.getElementById(eId);
  var s = e.style;
  this.interval = m_sec;
  this.target = e;
  s.position = 'absolute';
  Locator.call(s, e.offsetLeft, e.offsetTop);
  Starter.call(this, this.handrer);
};

Elevator.prototype.handrer = (function (D) {
  return function () {
    var t = this.target;
    var offsetX = D.clientWidth - t.offsetWidth;
    var offsetY = D.clientHeight - t.offsetHeight -50;
    var pointerX = /*@if (@_jscript) D.scrollLeft @else@*/ window.pageXOffset /*@end@*/ + offsetX;
    var pointerY = /*@if (@_jscript) D.scrollTop  @else@*/ window.pageYOffset /*@end@*/ + offsetY;
    var x = t.offsetLeft+((pointerX - t.offsetLeft)>>2);
    var y = t.offsetTop+((pointerY - t.offsetTop)>>2);
    var op = y - offsetY;
    if (op>0) Decorator.call(t.style,op>100?100:op);
    Locator.call (t.style, x, y);
  };
})(document[document.compatMode == 'CSS1Compat' ? 'documentElement' : 'body']);

//____

new Elevator('menu1', 50);

</script>