window.onload比$(document).ready加载时间要长，onload要加载dom，还要加载页面元素。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>onload和ready</title>
<script src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
</head>
<body>
<h3>onload和ready的先后测试</h3>
<script>
window.onload = function(){ alert('我是onload')}
$(document).ready(function(){alert('我是ready')});
</script>
</body>
</html>