div拖拽（用户拖动）

不能放视频，直接看代码吧
`// 拖拽事件
function tuozhuai(e,left,top){
// 获取盒子宽
var width = a.offsetWidth;
var height = a.offsetHeight;
//获取x坐标和y坐标
var nowX = e.clientX;
var nowY = e.clientY;
document.onmousemove = function(e) {
// 需要移动的 x y
var nx = e.clientX - nowX;
var ny = e.clientY - nowY;
//计算移动后的左偏移量和顶部的偏移量
var nl = left + nx;
var nt = top + ny;
// 判断出界
nl = nl + width > boxWidth?boxWidth - width:nl;
nl = nl < 0 ?0:nl;
nt = nt + height > boxHeight?boxHeight - height:nt;
nt = nt < 0 ?0:nt;
// 设置盒子left top
a.style.left = nl + 'px';
a.style.top = nt + 'px';
}
}