JSの最初につまづいたところ

JavaScript 
	<script type="text/javascript">
	// 一度HTMLを走査させてからじゃないとダメ
	window.onload = function () {
		// onChangeのように大文字がダメ
		// aに（）括弧つけるとダメ
		document.form1.select1.onchange = a
		function a () {
			// selectedIndexを変数にかましてやらないとダメ
			var number = document.form1.select1.selectedIndex
			// optionにするとダメ
			var className = document.form1.select1.options[number].text
			// これではIEで動かない
			document.body.setAttribute("class", className)
		}
	}
	</script>

</head>
<body id="top_page" class="C2_mainL_navL">

<div id="wrap">
	<!-- header -->
	<div id="header">
		header
		<form action="get" name="form1">
			<select name="select1">
				<option>C3_navF_extraF_mainL</option>
				<option>C3_navF_mainL_extraL</option>
			</select>
		</form>
	</div>

↓

window.addEventListener("load", setupEvents, false)

function setupEvents () {
	document.form1.select1.onchange = selectChange
	setAlpha()
}

function selectChange () {
	var target = document.form1.select1
	var number = target.selectedIndex
	//alert(number)
	var className = target.options[number].text
	document.body.setAttribute("class", className)
}

function setAlpha () {
	document.form1.select1.style.filter = "alpha(opacity=70)"
	document.form1.select1.style.MozOpacity = 0.7
	document.form1.select1.style.opacity = 0.7
}