<div class="box" id="box">
 <ul id="leftBox">
  <li class="active">a</li>
  <li>b</li>
  <li>c</li>
  <li>d</li>
 </ul>
 <div id="rightBox">
  <div id="rightBox1" class="active">
  <a href="#" >a1</a>
  <ul class="tabUl">
   <li class="hover">a1</li>
   <li>a2</li>
   <li>a3</li>
   <li>a4</li>
  </ul>
  </div>
  <div id="rightBox2">
  <a href="#" >b1</a>
  <ul class="tabUl">
   <li class="hover">b1</li>
   <li>b2</li>
   <li>b3</li>
   <li>b4</li>
  </ul>
  </div>
  <div id="rightBox3">
  <a href="#" >c1</a>
  <ul class="tabUl">
   <li class="hover">c1</li>
   <li>c2</li>
   <li>c3</li>
   <li>c4</li>
   <li>c5</li>
   <li>c6</li>
  </ul>
  </div>
  <div id="rightBox4">
  <a href="#" >d1</a>
  <ul class="tabUl">
   <li class="hover">d1</li>
   <li>d2</li>
   <li>d3</li>
   <li>d4</li>
  </ul>
  </div>
 </div>
</div>

<style>
 * {
  margin: 0px;
  padding: 0px;
  list-style: none;
  text-decoration: none;
 }

 .box {
  width: 800px;
  border: 1px solid #000000;
  margin: 20px auto;
  background: blue;
 }

 .box:after {
  content: "";
  display: block;
  clear: both;
 }

 #leftBox {
  float: left
 }

 #rightBox {
  float: left;
 }

 #leftBox li {
  width: 200px;
  height: 89px;
  background: red;
  margin-bottom: 2px;
  color: #fff;
  font: 50px/89px "黑体";
  text-align: center;
 }

 #rightBox div {
  display: none;
 }

 #rightBox .active {
  display: block;
 }

 #rightBox a {
  display: block;
  width: 600px;
  height: 325px;
  background: #0F0;
  font-size: 100px;
  color: #000;
  text-align: center;
  text-decoration: none;
  line-height: 360px;
 }

 .tabUl {
  display: table;
  width: 100%;
 }

 .tabUl li {
  display: table-cell;
  background: #909;
  color: #fff;
  font-size: 20px;
  text-align: center;
  height: 40px;
  line-height: 40px;
  border-right: 2px solid #03C;
  cursor: pointer;
 }

 .tabUl li.hover {
  background: #fff;
  color: #000;
 }

 #leftBox .active {
  background: yellow;
  color: #000;
 }
</style>

<script>
 /* 
 点击左边的li后，所点击li加上class名，其它清除所有，然后右边对应索引值的div也加上class=active，所以要用到this

  */

 //获取元素
 var lUl = document.getElementById('leftBox')
 var rUl = document.getElementById('rightBox')
 var lLi = lUl.getElementsByTagName('li')
 var oDiv = rUl.getElementsByTagName('div')

 //改变左边样式
 for (var i = 0; i < lLi.length; i++) {
  //自定义索引
  lLi[i].index = i
  lLi[i].onmouseenter = function () {
  //清空所有classname
  for (var j = 0; j < lLi.length; j++) {
   lLi[j].className = ''
   oDiv[j].className = ''
  }
  //给相应的li添加上classname
  this.className = 'active'
  oDiv[this.index].className = 'active'
  }
 }


 //右边
 for (var k = 0; k < oDiv.length; k++) {
  fn(oDiv[k])
 }
 function fn(parent) {
  //获取用到的元素
  var rLi = parent.getElementsByTagName('li')
  var oa = parent.getElementsByTagName('a')[0]
  for (var i = 0; i < rLi.length; i++) {
   rLi[i].onmouseenter = function(){
   for (j = 0; j < lLi.length; j++) {
    rLi[j].className = ''
   }
   this.className = 'hover'
   //把li标签中的文字传到a标签中
   oa.innerHTML = this.innerHTML
   }
  
  }
  
 }
</script>