onload事件是整個頁面加載完以後，執行的事件。注意，在onload裏面爲事件指定函數名稱時候不可以加括號
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style>
#dv{
width: 100px;
height: 100px;
background-color: grey;
}
</style>
<script>
window.onload=function(){
var dv = document.getElementById('dv');
dv.onmouseover=togreen
dv.onmouseout=togrey
}
function togreen(){
dv.style.height='200px';
dv.style.width='200px';
dv.style.backgroundColor='red';
}
function togrey(){
dv.style.height='100px';
dv.style.width='100px';
dv.style.backgroundColor='grey';
}
</script>
</head>
<body>
<div id='dv'>hello</div>
</body>
</html>