Essayez-le vous-même - 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 uses the HTML DOM to assign "ondragstart", "ondragenter", "ondragleave", "ondragover" and "ondrop" events to the document object.</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>

/* Events fired on the drop target */
document.ondragenter = function(event) {
    if ( event.target.className == "droptarget" ) {
        document.getElementById("demo").innerHTML = "Entered the dropzone";
        event.target.style.backgroundColor = "lightblue";
    }
};

document.ondragover = function(event) {
    event.preventDefault();
};

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

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

</body>
</html>