为嘛我这个鼠标拖拽效果出不来？

Question

HTML:<div id=box>  <img src="images/psb.jpg" alt="师大光影" id="img1"/>   <img src="images/psb.jpg" alt="师大光影" id="img2"/>   <div id=main>      <div id="left-up" class="minDiv left-up"></div>      <div id="upDiv" class="minDiv up"></div>      <div id="right-up"  class="minDiv right-up"></div>      <div id="leftDiv" class="minDiv left"></div>      <div id= rightDiv class="minDiv right"></div>      <div id="left-bottom" class="minDiv left-bottom"></div>      <div id="downDiv" class="minDiv bottom"></div>      <div id="right-bottom" class="minDiv right-bottom"></div>   </div>js代码:mainDiv.ommousedown=function(){    var boxDiv=document.getElementById("box");     var mainDiv=document.getElementById("main");    boxDiv.onmousemove= function(event){     event=event||window.event;     var x=event.clientX;     var y=event.clientY;        mainDiv.style.top=y+"px";     mainDiv.style.left=x+"px";   }  }