Prova tu stesso - JavaScript Reference ondragover

<!DOCTYPE HTML>
<html>
<head>
<style>
#droptarget {
   float: left;
   width: 200px;
   height: 35px;
   margin: 55px;
   margin-top: 155px;
   padding: 10px;
   border: 1px solid #aaaaaa;
}
</style>
</head>
<body>

<p ondragstart="dragStart(event)" draggable="true" id="dragtarget">Drag me into the rectangle!</p>

<div id="droptarget" ondrop="drop(event)" ondragover="allowDrop(event)">
</div>

<p style="clear:both;"><strong>Note:</strong> drag events are not supported in Internet Explorer 8 and earlier versions or Safari 5.1 and earlier versions.</p>

<p id="demo"></p>

<script>
function dragStart(event) {
   event.dataTransfer.setData("Text", event.target.id);
}

function allowDrop(event) {
   event.preventDefault();
   document.getElementById("demo").innerHTML = "The p element is OVER the droptarget.";
   event.target.style.border = "4px dotted green";
}

function drop(event) {
   event.preventDefault();
   var data = event.dataTransfer.getData("Text");
   event.target.appendChild(document.getElementById(data));
   document.getElementById("demo").innerHTML = "The p element was dropped.";
}
</script>

</body>
</html>

<p ondragstart="dragStart(event)" draggable="true" id="dragtarget">Drag me into the rectangle!</p>

<p style="clear:both;"><strong>Note:</strong> drag events are not supported in Internet Explorer 8 and earlier versions or Safari 5.1 and earlier versions.</p>

function allowDrop(event) {
    event.preventDefault();
    document.getElementById("demo").innerHTML = "The p element is OVER the droptarget.";
    event.target.style.border = "4px dotted green";
}

function drop(event) {
    event.preventDefault();
    var data = event.dataTransfer.getData("Text");
    event.target.appendChild(document.getElementById(data));
    document.getElementById("demo").innerHTML = "The p element was dropped.";
}
</script>

</body>
</html>