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>