<div id="demo" style="width:100px; height:100px; position:absolute; background-color:silver;"></div>
<script>
function dragable(id){var d=document,o=d.getElementById(id),s=o.style,x,y,p='onmousemove';o.onmousedown=function(e){e=e||event;x=e.clientX-o.offsetLeft;y=e.clientY-o.offsetTop;d[p]=function(e){e=e||event;s.left=e.clientX-x+'px';s.top=e.clientY-y+'px'};d.onmouseup=function(){d[p]=null}}}
dragable("demo");
</script>