</style>
   <script>
    window.onload = function(){
       var container = document.getElementById('container');
    var listImg = document.getElementById('listImg');
    var buttons = document.getElementById('buttons');
    var next = document.getElementById('next');
    var prev = document.getElementById('prev');
    var index = 1;
    function animate(offset){
         var newlist = parseInt(listImg.style.left)+offset;
      listImg.style.left = newlist + 'px';
     if(newlist > -700){
         listImg.style.left = -3500 + 'px';
     }
     if(newlist < -3500){
         listImg.style.left = -700 + 'px';
     }
    }
    function showButtons(){
        buttons[index-1].className = 'on';     在这用className浏览器报  ‘Uncaught TypeError: Cannot set property 'className' of undefined ’ 这个错呢
    }

    next.onclick = function(){
        showButtons();
        animate(-700);
    }
    prev.onclick = function(){
        animate(700);
    }
    }
   
   </script>