classname 就是在css上添加类，然后js的类名等于

	<!DOCTYPE HTML>
	<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
	<title>无标题文档</title>
	<script>
	window.onload = function (){
	var oBtn1 = document.getElementById('btn1');
	var oBtn2 = document.getElementById('btn2');
	var oBtn3 = document.getElementById('btn3');
	var oBtn4 = document.getElementById('btn4');
	var oP = document.getElementById('p1');
	var num = 14;
	oBtn1.onclick = function (){
	num -= 2;
	oP.style.fontSize = num + 'px';
	// JS 不允许出现"-"
	// padding-top paddingTop
	// margin-left marginLeft
	};
	oBtn2.onclick = function (){
	// num = num + 2;
	num += 2;
	oP.style.fontSize = num + 'px';
	// JS 不允许出现"-"
	};
	oBtn3.onclick = function (){
	// oP.class = 'red';
	// class => className
	oP.className = 'red';
	};
	oBtn4.onclick = function (){
	// oP.class = 'red';
	// class => className
	oP.className = 'yellow';
	};
	};
	</script>
	<style>
	.red { width:400px; border:10px solid #333; background:red; padding:20px; color:yellow; }
	.yellow { width:500px; border:5px solid #333; background:yellow; padding:10px; color:red; }
	</style>
	</head>
	<body>
	<input id="btn1" type="button" value="-" />
	<input id="btn2" type="button" value="+" />
	<input id="btn3" type="button" value="红" />
	<input id="btn4" type="button" value="黄" />
	<p id="p1" style="font-size:16px;">10月28日晚，中央纪委监察部官网发布消息，贵州省委常委、遵义市委书记廖少华因涉嫌严重违纪违法接受组织调查。3天后中组部宣布对其免职。廖成为十八大后中纪委一连串"打虎"行动中第十一位落马的副省部级以上高官。</p>
	</body>
	</html>