required="required" -> 필수입력 속성

                                                              <!-- required반드시써야함 -->
<tr> <td>ID</td> <td><input type="text" placeholder="id를 입력하세요" required="required"> </td></tr> 
<tr> <td> 비밀번호 </td> <td><input type="password" name="pwd" id="pwd" required> </td></tr>