<ul class="list" id="list">
  <li class="in">1</li>
  <li class="in">2</li>
  <li class="in">3</li>
  <li class="in">4</li>
  <li class="in">5</li>
  <li class="in">6</li>    
</ul>

.in{
  height: 20px;
  line-height: 20px;
  width: 20px;
  background-color: blue;
  text-align: center;
  color: white;
}

<script>
var oList = document.getElementById('list');
//新增一个li元素
var oAdd = document.createElement('li');
//设置新增元素的css样式
oAdd.className = "in";
oAdd.style.cssText = 'background-color:red;border-radius:50%';
//1s后oAdd替换第0个li
setTimeout(function(){
  oList.replaceChild(oAdd,document.getElementsByTagName('li')[0]);
  //1s后执行incrementNumber函数
  setTimeout(incrementNumber,1000);  
},1000);
function incrementNumber(){
  //获取oList中第1个li
  var oLi1 = document.getElementsByTagName('li')[1];
  //若存在则进行替换处理
  if(oLi1){
     oList.replaceChild(oAdd,oLi1);
     setTimeout(incrementNumber,1000);    
  }
}
</script>