<form action="#" method="post">
          <input type="text" name="username" class="username" placeholder="请输入您的用户名！">
          <input type="password" name="password" class="password" placeholder="请输入您的用户密码！">
          <input type="Captcha" class="Captcha" name="Captcha" placeholder="请输入验证码！">
          <button type="submit" class="submit_button">登录</button>
          <div class="error"><span>+</span></div>
</form>

<button type="submit" class="submit_button" onClick=“submitAction();return false;”>登录</button>