html运行实例（event_ondragenter

代码示例：（标识：event_ondragenter_html）
<!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>本例演示如何将 "ondragstart" 事件分配给 p 元素，并将 "ondragenter"、"ondragleave"、"ondragover" 和 "ondrop" 事件分配给 div 元素。</p>

<p>在两个矩形之间来回拖动 p 元素：</p>

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

<div class="droptarget" ondragenter="dragEnter(event)" ondragleave="dragLeave(event)" ondrop="drop(event)" ondragover="allowDrop(event)"></div>

<p style="clear:both;"><b>注释：</b>Internet Explorer 8 及更早版本或 Safari 5.1 及更早版本不支持 drag 事件。</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>