css :not(selector) 选择器不生效

以下两种情况:not()选择器均不生效

<div class="nav">
  <div></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

<style>
.nav .item:not(:first-child) {    
   margin-top: 20px;
}
</style>

:first-child 列表元素前面有元素不生效

<div class="nav">
  <div class="item"></div>
  <div class="item"></div>
  <div></div>
</div>

<style>
.nav .item:not(:last-child) {    
  border-bottom: 1px solid #666;  
}
</style>

:last-child 列表元素后面有元素不生效

解决:
将列表元素用div包起来

<div class="nav">
  <div>
    <div class="item"></div>
    <div class="item"></div>
  </div>
</div>