window.onload = function(){
var width = document.getElementById("123").style.width;
var height = document.getElementById("123").style.height;
document.write(width);
document.getElementById("123").style.backgroundSize = "Math.max(width, height), Math.max(width, height)";
}
我不知道最后一句设置背景图片大小的应该怎么写，我是想让背景图片要能够刚好覆盖div
