IE 的事件机制中，触发事件会从子元素向父元素逐级上传，如果子元素触发了单击事件，那么也会触发父元素的单击事件；event.cancelBubble=true;可以停止事件继续上传。补充一点，ie的事件传递是从下到上的：
事件来源对象->上级对象->上上级对象->.....->body->document->window
NS的事件传递是从上到下：
window->document->body->....->事件来源对象实例源码如下：
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>event.cancelBubble</title>
<style>
<!--
* {font:menu}
-->
</style>
</head>
<body>
<span onclick=alert("你好")>点我 <span>再点我</span></span><br><br>
<span onclick=alert("你好")>点我 <span onclick=event.cancelBubble=true;>再点我</span></span>
</body>
</html>
实例2：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="zh" xml:lang="zh">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta name="developer" content="Realazy" />
<title>Bubble in JavaScript DOM--JavaScript 的事件冒泡 demo </title>
<style type="text/css" media="screen">
div * {display:block; margin:4px; padding:4px; border:1px solid white;}
textarea {width:20em; height:2em;}
</style>
<script type="text/javascript">
//<![CDATA[
function init(){
var log = document.getElementsByTagName('textarea')[0];
var all = document.getElementsByTagName('div')[0].getElementsByTagName('*');
for (var i = 0, n = all.length; i < n; ++i){
all[i].onmouseover = function(e){
this.style.border = '1px solid red';
log.value = '鼠标现在进入的是： ' + this.nodeName;
};
all[i].onmouseout = function(e){
this.style.border = '1px solid white';
};
}
var all2 = document.getElementsByTagName('div')[1].getElementsByTagName('*');
for (var i = 0, n = all2.length; i < n; ++i){
all2[i].onmouseover = function(e){
this.style.border = '1px solid red';
if (e) //停止事件冒泡
e.stopPropagation();
else
window.event.cancelBubble = true;
log.value = '鼠标现在进入的是： ' + this.nodeName;
};
all2[i].onmouseout = function(e){
this.style.border = '1px solid white';};
}
}
window.onload = init;
//]]>
</script>
</head>
<body>
<h1>Bubble in JavaScript DOM</h1>
<p>DOM树的结构是：</p>
<pre><code>
UL
- LI
- A
- SPAN
</code></pre>
<div>
<ul>
<li><a href="#"><span>Bubbllllllllllllllle</span></a></li>
<li><a href="#"><span>Bubbllllllllllllllle</span></a></li>
</ul>
</div>
<textarea></textarea>
<p>鼠标进入UL的任何一个子元素，如果不停止冒泡，我们从UL到SPAN都定义了鼠标悬停（<code>mouseover</code>）事件，这个事件会上升了UL，从而从鼠标所进入的元素到UL元素都会有红色的边。</p>
<div>
<ul>
<li><a href="#"><span>Bubbllllllllllllllle</span></a></li>
<li><a href="#"><span>Bubbllllllllllllllle</span></a></li>
</ul>
</div>
<p>如果停止冒泡，事件不会上升，我们就可以获取精确的鼠标进入元素。</p>
</body>
</html>