事件会在鼠标按键被按下时发生。
支持该事件的 HTML 标签：
<a>, <address>, <area>, <b>, <bdo>, <big>, <blockquote>, <body>, <button>,
<caption>, <cite>, <code>, <dd>, <dfn>, <div>, <dl>, <dt>, <em>, <fieldset>,
<form>, <h1> to <h6>, <hr>, <i>, <img>, <input>, <kbd>, <label>, <legend>,
<li>, <map>, <ol>, <p>, <pre>, <samp>, <select>, <small>, <span>, <strong>,
<sub>, <sup>, <table>, <tbody>, <td>, <textarea>, <tfoot>, <th>, <thead>,
<tr>, <tt>, <ul>, <var>
支持该事件的 JavaScript 对象：
button, document, link
例子：
显示所点击元素的标签名：
<html>
<head>
<script type="text/javascript">
function alertElement(e)
{
var targ;
if (!e){
var e = window.event;
}
if (e.target){
targ = e.target;
}
else if (e.srcElement){
targ = e.srcElement;
}
if (targ.nodeType == 3){ // defeat Safari bug
targ = targ.parentNode;
}
var tname;
tname=targ.tagName;
alert("你所点击的标签元素为：" + tname);
}
</script>
</head>
<body onmousedown="alertElement(event)">
<span>span元素</span>
<p>p元素</p>
<div>div元素</div>
</body>
</html>