Determine the number of checkbox selections multiple browsers_javascript skills

You cannot allow users to select more than 5 checkboxes

<title> New Document </title> <meta name="Generator" content="EditPlus"> <meta name="Author" content=""> <meta name="Keywords" content=""> <meta name="Description" content=""> <table cellspacing="0" border="1" width="200"> <tr> <td><input type="checkbox" name="c"></td> <td><input type="checkbox" name="c"></td> <td><input type="checkbox" name="c"></td> </tr> <tr> <td><input type="checkbox" name="c"></td> <td><input type="checkbox" name="c"></td> <td><input type="checkbox" name="c"></td> </tr> <br><input onclick="runEx('runcode24277')" type="button" value="运行代码"> <input onclick="doCopy('runcode24277')" type="button" value="复制代码"> <input onclick="doSave(runcode24277)" type="button" value="保存代码">[Ctrl A 全选 注:<a href="http://www.jb51.net/article/23421.htm" title="查看具体详情" target="_blank">如需引入外部Js需刷新才能执行</a>]<br><strong>第二种方法<br></strong><div class="htmlarea"><textarea id="runcode15991"> <meta http-equiv="Content-Type" content="text/html; charset=gb2312"> <title></title>