问如果输入框为空，则在提交时使用jQuery addClass

EN

<div id="form_div">
  <form id="form_id">
    <fieldset id="fieldset">
      <div>
        <label for="fname">First Name:</label>
        <input type="text" id="fname" name="fname">
      </div>
      <div>
        <label for="lname">Last Name:</label>
        <input type="text" id="lname" name="lname">
        <p class="error_msg">You must enter a last name</p>
      </div>
</fieldset>
    <br>
    <button type="submit" id="btn_submit">Submit</button>
  </form>
</div>

.error_msg {
  display: inline !important;
  color: red;
  font-size: 13px;
  margin-left: 1em !important;
}

  $("#form_div").submit(function(evt) {
    if($('#lname').val() === ''){
      $("#lname").addClass("error_msg");
    }
  });