:hidden：表示查询所有隐蔽的标签
:visible：表示查询所有显示的标签
或:not(:hidden)
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<script type="text/javascript" src="../js/jquery-1.6.js"></script>
</head>
<body>
<table border="1" align="center">
<tr >
<td>Value 1</td>
</tr>
<tr>
<td>Value 2</td>
</tr>
<tr>
<td>Value 3</td>
</tr>
</table>
<script type="text/javascript">
//1)查找隐藏的tr元素的个数
//alert($("table tr:hidden").size());
//alert($("tr:hidden").size());
//2)查找所有可见的tr元素的个数
//alert($("table tr:visible").size());
//alert($("table tr:not(:hidden)").size());
</script>
</body>
</html>