Prova tu stesso - JavaScript Reference ondragenter

<!DOCTYPE HTML>
<html>
<head>
<style>
.droptarget {
   float: left;
   width: 100px;
   height: 35px;
   margin: 15px;
   margin-right: 100px;
   padding: 10px;
   border: 1px solid #aaaaaa;
}
</style>
</head>
<body>

<p>This example demonstrates how to assign an "ondragstart" event to a p element and an "ondragenter", "ondragleave", "ondragover" and "ondrop" event to a div element.</p>

<p>Drag the p element back and forth between the two rectangles:</p>

<div class="droptarget" ondrop="drop(event)" ondragenter="dragEnter(event)" ondragleave="dragLeave(event)" ondragover="allowDrop(event)">
  <p ondragstart="dragStart(event)" draggable="true" id="dragtarget">Drag me!</p>
</div>

<div class="droptarget" ondragenter="dragEnter(event)" ondragleave="dragLeave(event)" 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 dragEnter(event) {
   if ( event.target.className == "droptarget" ) {
   document.getElementById("demo").innerHTML = "Entered the dropzone";
   event.target.style.backgroundColor = "lightblue";
   }
}

function dragLeave(event) {
   if ( event.target.className == "droptarget" ) {
   document.getElementById("demo").innerHTML = "Left the dropzone";
   event.target.style.backgroundColor = "";
   }
}

function allowDrop(event) {
   event.preventDefault();
}

function drop(event) {
   event.preventDefault();
   event.target.style.backgroundColor = "";
   var data = event.dataTransfer.getData("Text");
   event.target.appendChild(document.getElementById(data));
}
</script>

</body>
</html>

<p>This example demonstrates how to assign an "ondragstart" event to a p element and an "ondragenter", "ondragleave", "ondragover" and "ondrop" event to a div element.</p>

<p>Drag the p element back and forth between the two rectangles:</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 dragEnter(event) {
    if ( event.target.className == "droptarget" ) {
        document.getElementById("demo").innerHTML = "Entered the dropzone";
        event.target.style.backgroundColor = "lightblue";
    }
}

function dragLeave(event) {
    if ( event.target.className == "droptarget" ) {
        document.getElementById("demo").innerHTML = "Left the dropzone";
        event.target.style.backgroundColor = "";
    }
}

function allowDrop(event) {
    event.preventDefault();
}

function drop(event) {
    event.preventDefault();
    event.target.style.backgroundColor = "";
    var data = event.dataTransfer.getData("Text");
    event.target.appendChild(document.getElementById(data));
}
</script>

</body>
</html>