怎么样读剪贴板?

<HEAD>
<SCRIPT>
function InitiateDrag(){
  event.dataTransfer.setData(oSource.innerText);
}function FinishDrag(){
  window.event.returnValue=false;
  oTarget.innerText = event.dataTransfer.getData("Text");
}
function OverDrag(){
  window.event.returnValue=false;
}
</SCRIPT>
</HEAD>
<BODY><B ID="oSource"
   ondragstart="InitiateDrag()">
drag this text</B><SPAN ID="oTarget"
   ondragover="OverDrag()"
   ondragenter="FinishDrag()"">
drop text here</SPAN></BODY>