急。。。一个表单的问题

<%@ page contentType="text/html; charset=utf-8" language="java" import="java.sql.*" errorPage="" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>留言板系统</title>
<script language="javascript" type="text/javascript">
function test(){
alert(OK);
}
</script>
</head><body>
  <center>
   <div>
     留言板
    </div>
    <div>
      <form method="post" action="">
     <table align="center">
         <tr>
             <td>
                 用户名：
                </td>
             <td>
                 <input type="text" /><font color="#FF0000">*</font>
                </td>
            </tr> 
            <tr>
             <td>
                 标题：
                </td>
             <td>
                 <input type="text" /><font color="#FF0000">*</font>
                </td>
            </tr>
            <tr>
             <td>
                 字体大小：
                </td>
                <td>
                 <select>
                     <option value="0">0</option>
                        <option value="1">1</option>
                        <option value="2">2</option>
                        <option value="3" selected="selected">3</option>
                        <option value="4">4</option>
                        <option value="5">5</option>
                        <option value="6">其他</option>
                    </select>
                </td>
            </tr>
            <tr>
             <td colspan="2" align="center">
                 留言内容
                </td>
            </tr>
            <tr>
             <td colspan="2">
                <textarea cols="30" rows="10"></textarea>
                </td>
            </tr>
            <tr>
             <td>
                 <input type="button" value="清空内容" onclick="test()"/>
                </td>
                <td>
                 <input type="submit" value="发表留言"/>
                </td>
            </tr>  
            <tr>
             <td colspan="2" align="center">
                带<font color="#FF0000">*</font>的为必填项
                </td>
            </tr>     
        </table>
        </form>
    </div>
  </center>
</body>
</html>
写了一个简易的留言板，想用一个js清空textarea中的内容，在测试按钮触发js的时候出问题了，怎么点都没有反应