思路：
利用onmousedown事件实现拖拽。首先获得鼠标横坐标点和纵坐标点到div的距离，然后当鼠标移动后再用可视区的距离减去横纵坐标与div的距离。然后在判断不让DIV移出可视区，然后再赋予DIV的位置。最后关闭鼠标onmouseup事件。
<!DOCTYPE html>
<html lang="en">
<head runat="server">
<title></title>
<style type="text/css">
#div1 {
width: 200px;
height: 200px;
background: #00FFFF;
position: absolute;
border: 1px solid;
}
</style>
<script type="text/javascript">
window.onload = function () {
var div = document.getElementById('div1');
var disX = 0;
var disY = 0;
div.onmousedown = function (ev) { //鼠标按下
var oEvent = ev || event; //判断浏览器兼容
disX = oEvent.clientX - div1.offsetLeft; //鼠标横坐标点到div的offsetLeft距离
disY = oEvent.clientY - div1.offsetTop; //鼠标纵坐标点到div的offsetTop距离
div.onmousemove = function (ev) { //鼠标移动
var oEvent = ev || event;
var l = oEvent.clientX - disX; //获取div左边的距离
var t = oEvent.clientY - disY; //获取div上边的距离
if (l < 0) { //判断div的可视区，为避免DIV失去鼠标点
l = 0;
}
else if (l > document.documentElement.clientWidth - div.offsetWidth) {
l = document.documentElement.clientWidth - div.offsetWidth;
}
if (t < 0) {
t = 0;
}
else if (t > document.documentElement.clientHeight - div.offsetHeight) {
t = document.documentElement.clientHeight - div.offsetHeight;
}
div.style.left = l + 'px'; //确定DIV的左边位置
div.style.top = t + 'px'; //确定DIV的上边位置
}
div.onmouseup = function () { //当鼠标松开后关闭移动事件和自身事件
div.onmousemove = null;
div.onmouseup = null;
}
return false;
}
}
</script>
</head>
<body>
<div id="div1">
</div>
</body>
</html>
