老师，解释下

<template>
<div>
<div 
class="item"
v-for="(item,index) of list"
:key="index">
  <div class="item-title border-bottom">
   <span class="item-title-icon"></span>
   {{item.title}}
  </div>
  <div v-if="item.children" class="item-children">  <!-- 如果当前的一级列表下有children ，就将children也显示在二级菜单那里-->
   <detail-list :list="item.children"></detail-list>
  </div>
</div>
</div>
</template>

<script>
export default {
name: 'DetailList',
props: {
list:Array
}
}
</script>

<style scoped>
  .item-title-icon
    position:relative
    left:.06rem
    top:.06rem
    display:inline-block
    width:.36rem
    height:.36rem
    background:url(http://s.qunarzz.com/piao/image/touch/sight/detail.png) 0 -.45rem no-repeat
    background-size:.4rem
  .item-title
   line-height:.8rem
   font-size:.32rem
   padding: 0 .2rem
  .item-children
   padding:0 .2rem
</style>

这里的.item-title-icon样式为什么不能写在.item-title的后面，会报错，是因为html结构的原因？？