                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /JS动画效果

onmouseover事件触发的疑惑？

http://www.imooc.com/video/2879/0

这节课4分14秒的代码，明明”分享“已经在父盒子div1之外了，将div1绑定onmouseover事件后，为什么鼠标移动到”分享“上仍然可以触发？？而类似的下面的这段代码却无法触发alert指令？？

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title></title>
  <style type="text/css">
  *{
   margin: 0;
   padding: 0;
  }
   .container #share{
    width: 20px;
    height: 50px;
    background: blue;
    position: absolute;
    left:200px;
    top:75px;
   }
   .container{
    width: 200px;
    height: 200px;
    background: red;
    position: relative;
    left: -200px;
    top: 0px;
   }
  </style>
  <script type="text/javascript">
   window.onload=function (){
    var container = document.getElementsByClassName('container')
    container.onmouseover= function (){
     startMove();
    }
   }
   function startMove(){
    alert(123);
   }
  </script>
</head>
<body>
  <div class="container">
   <span id="share">
    分享
   </span>
  </div>
</body>
</html>