当然，也有这方面很炫的代码，但是才跨浏览器方面功夫还是不够，呵呵。我遇到一个不错的css，在IE和Firefox下浏览一样的效果。

      下图是实际运行的效果：

代码如下：

/******************************************************

  dw_viewport.js
  version date Nov 2003
 
  This code is from Dynamic Web Coding
  at http://www.dyn-web.com/
  Copyright 2003 by Sharon Paine
  See Terms of Use at http://www.dyn-web.com/bus/terms.html
  regarding conditions under which you may use this code.
  This notice must be retained in the code as is!

*******************************************************/ 
 
var viewport = {
  getWinWidth: function () {
    this.width = 0;
    if (window.innerWidth) this.width = window.innerWidth - 18;
    else if (document.documentElement && document.documentElement.clientWidth)
    this.width = document.documentElement.clientWidth;
    else if (document.body && document.body.clientWidth)
    this.width = document.body.clientWidth;
  },
 
  getWinHeight: function () {
    this.height = 0;
    if (window.innerHeight) this.height = window.innerHeight - 18;
   else if (document.documentElement && document.documentElement.clientHeight)
    this.height = document.documentElement.clientHeight;
   else if (document.body && document.body.clientHeight)
    this.height = document.body.clientHeight;
  },
 
  getScrollX: function () {
    this.scrollX = 0;
   if (typeof window.pageXOffset == "number") this.scrollX = window.pageXOffset;
   else if (document.documentElement && document.documentElement.scrollLeft)
    this.scrollX = document.documentElement.scrollLeft;
   else if (document.body && document.body.scrollLeft)
    this.scrollX = document.body.scrollLeft;
   else if (window.scrollX) this.scrollX = window.scrollX;
  },
 
  getScrollY: function () {
    this.scrollY = 0;   
    if (typeof window.pageYOffset == "number") this.scrollY = window.pageYOffset;
    else if (document.documentElement && document.documentElement.scrollTop)
    this.scrollY = document.documentElement.scrollTop;
   else if (document.body && document.body.scrollTop)
    this.scrollY = document.body.scrollTop;
   else if (window.scrollY) this.scrollY = window.scrollY;
  },
 
  getAll: function () {
    this.getWinWidth(); this.getWinHeight();
    this.getScrollX();  this.getScrollY();
  }
 
}

/****************************************************
    dw_event.js (version date Feb 2004)
       
    This code is from Dynamic Web Coding at http://www.dyn-web.com/
    See Terms of Use at http://www.dyn-web.com/bus/terms.html
    regarding conditions under which you may use this code.
    This notice must be retained in the code as is!
*******************************************************/

var dw_event = {
 
  add: function(obj, etype, fp, cap) {
    cap = cap || false;
    if (obj.addEventListener) obj.addEventListener(etype, fp, cap);
    else if (obj.attachEvent) obj.attachEvent("on" + etype, fp);
  },

  remove: function(obj, etype, fp, cap) {
    cap = cap || false;
    if (obj.removeEventListener) obj.removeEventListener(etype, fp, cap);
    else if (obj.detachEvent) obj.detachEvent("on" + etype, fp);
  },

  DOMit: function(e) {
    e = e? e: window.event;
    e.tgt = e.srcElement? e.srcElement: e.target;
   
    if (!e.preventDefault) e.preventDefault = function () { return false; }
    if (!e.stopPropagation) e.stopPropagation = function () { if (window.event) window.event.cancelBubble = true; }
       
    return e;
  }
 
}
/**************************************************
  dw_tooltip.js   requires: dw_event.js and dw_viewport.js
  version date: March 14, 2005
  (minor changes in position algorithm and timer mechanism)
 
  This code is from Dynamic Web Coding at dyn-web.com
  Copyright 2003-5 by Sharon Paine
  See Terms of Use at www.dyn-web.com/bus/terms.html
  regarding conditions under which you may use this code.
  This notice must be retained in the code as is!
*****************************************************/

var Tooltip = {
    followMouse: true,
    offX: 8,
    offY: 12,
    tipID: "tipDiv",
    showDelay: 100,
    hideDelay: 200,
   
    ready:false, timer:null, tip:null,
 
    init: function() { 
        if ( document.createElement && document.body && typeof document.body.appendChild != "undefined" ) {
            if ( !document.getElementById(this.tipID) ) {
                var el = document.createElement("DIV");
                el.id = this.tipID; document.body.appendChild(el);
            }
            this.ready = true;
        }
    },
   
    show: function(e, msg) {
        if (this.timer) { clearTimeout(this.timer); this.timer = 0; }
        this.tip = document.getElementById( this.tipID );
        if (this.followMouse) // set up mousemove
            dw_event.add( document, "mousemove", this.trackMouse, true );
        this.writeTip("");  // for mac ie
        this.writeTip(msg);
        viewport.getAll();
        this.positionTip(e);
        this.timer = setTimeout("Tooltip.toggleVis('" + this.tipID + "', 'visible')", this.showDelay);
    },
   
    writeTip: function(msg) {
        if ( this.tip && typeof this.tip.innerHTML != "undefined" ) this.tip.innerHTML = msg;
    },
   
    positionTip: function(e) {
        if ( this.tip && this.tip.style ) {
            // put e.pageX/Y first! (for Safari)
            var x = e.pageX? e.pageX: e.clientX + viewport.scrollX;
            var y = e.pageY? e.pageY: e.clientY + viewport.scrollY;
   
            if ( x + this.tip.offsetWidth + this.offX > viewport.width + viewport.scrollX ) {
                x = x - this.tip.offsetWidth - this.offX;
                if ( x < 0 ) x = 0;
            } else x = x + this.offX;
       
            if ( y + this.tip.offsetHeight + this.offY > viewport.height + viewport.scrollY ) {
                y = y - this.tip.offsetHeight - this.offY;
                if ( y < viewport.scrollY ) y = viewport.height + viewport.scrollY - this.tip.offsetHeight;
            } else y = y + this.offY;
           
            this.tip.style.left = x + "px"; this.tip.style.top = y + "px";
        }
    },
   
    hide: function() {
        if (this.timer) { clearTimeout(this.timer); this.timer = 0; }
        this.timer = setTimeout("Tooltip.toggleVis('" + this.tipID + "', 'hidden')", this.hideDelay);
        if (this.followMouse) // release mousemove
            dw_event.remove( document, "mousemove", this.trackMouse, true );
        this.tip = null;
    },

    toggleVis: function(id, vis) { // to check for el, prevent (rare) errors
        var el = document.getElementById(id);
        if (el) el.style.visibility = vis;
    },
   
    trackMouse: function(e) {
     e = dw_event.DOMit(e);
      Tooltip.positionTip(e);
    }
   
}

Tooltip.init();