样式

  <style>
    /* 表单必填项，右红三角 */
    .fn_bixuan{position: relative;}
    .fn_bixuan::after{position: absolute;display: block;content: ''; z-index: 1;transform: rotate(-45deg); left: -10px;top: -10px;border: 10px solid; border-color: transparent transparent red;}
  </style>

html：在class中加fn_bixuan；不能加在表单元素本身上

//文本框
<div class="layui-input-block fn_bixuan">
	<input type="text" name="title" placeholder="标题"  autocomplete="off" class="layui-input">
</div>

//下拉框
<div class="layui-input-block fn_bixuan" >
	<select name="id">
		<option value="">请选择</option>
	</select>
</div>