var box = document.getElementById("box");
box.onmousedown = function(event) {
    // e.clientX	事件发生时，鼠标在客户端区域的X坐标，客户端区域是指页面可视区域
    // e.clientY	事件发生时，鼠标在客户端区域的Y坐标
    var left1 = event.clientX - box.offsetLeft;
    var top1 = event.clientY - box.offsetTop;
    box.onmousemove = function(e) {
        box.style.left = e.clientX - left1 + 'px';
        box.style.top = e.clientY - top1 + 'px';
    }
}
box.onmouseup = function() {
    box.onmousemove = null;
}