javascript获取css中的样式值

     <body>
         <input type="button" id="btn" value="启动"/>
         <img src="timg.jpg" id="img" alt="" />
         <script type="text/javascript">

              window.onload = function()
              {
                  var time = null;
                  var img = document.getElementById("img");

                 img.onmouseover = function()
                 {
                      startMove(img,100,10);

                    //alert(getComputedStyle(img,false).opacity)    

                 }
                  img.onmouseout = function()
                 {
                     startMove(img,30,-10);
                 }

                  function startMove(obj,iTarget,ispeed)
                {
                     var icur =0;
                       clearInterval(time);
                       time = setInterval(function(){
                             icur = Math.round((getComputedStyle(img,false).opacity)*100);  //为什么不能获取到呢
                               if(icur == iTarget)
                               {
                                   clearInterval(time);
                               }else{
                                    obj.style.opacity = (icur + ispeed)/100 ;
                                    obj.style.filter = 'alpha(opacity = '+(icur + ispeed)+')';
                               }
                       },30)
                }
              }

         </script>
     </body>

注意 ie下不支持  

getComputedStyle