自从上次写过拖拽就忘了怎么实现。今天又研究了半天，终于搞懂了。

首先，被拖拽的元素一定要设置position:absolute，今天就是忘了这一点，频频出错，不得要领。

其次，拖拽分三个步骤：

1.点击元素，即onmousedown;

2.移动，即onmousemove;

3.鼠标弹起，即onmouseup;

总结代码如下：

pox.onmousedown = function(e){

    var e = e || window.event;

    var diffX = e.clientX - pox.offsetLeft;

    var diffY = e.clientY - pox.offsetTop;

    document.onmousemove = function(e){

        var e = e || window.event;

        pox.style.left = e.clientX - diffX + 'px';

        pox.style.top = e.clientY - diffY + 'px';

    }

    document.onmouseup = function(){

        document.onmousemove = null;

        document.onmouseup = null;

    }

}