这是自己犯的低级错误：
css:
<link rel="stylesheet" href="../css/test.css">
<link href="../css/test.css"> <!-- 没指明rel，导致样式引入失败 -->jquery插件的引用：
<script type="text/javascript" src="../js/jquery.min.js"></script>
<script type="text/javascript" src='../js/leftnav.js'></script>错误：在jquery之前导入了基于其的js文件
<script type="text/javascript" src='../js/leftnav.js'>
</script> <!-- 此处会报'$'is not found -->
<script type="text/javascript" src="../js/jquery.min.js"></script>