给P增加一个属性，通过属性来判断当前状态
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>display</title>
<script type="text/javascript">
function changeText()
{
var mychar = document.getElementById("con");
if(mychar.getAttribute('show') == "yes"){
mychar.setAttribute('show', "no");
mychar.style.display = 'none'
}else{
mychar.setAttribute('show', "yes")
mychar.style.display = 'block'
}
}
</script>
</head>
<body>
<h1>JavaScript</h1>
<p id="con" show = "yes">做为一个Web开发师来说，如果你想提供漂亮的网页、令用户满意的上网体验，JavaScript是必不可少的工具。</p>
<form>
<input type="button" onclick="changeText()" value="改变内容" />
</form>
</body>
</html>