如何指定一个层将整个客户端的屏幕撑满？
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参考：
div.style.position = "absolute ";
div.style.left = 0;
div.style.top = 0;
div.style.width = (document.body.scrollWidth> document.body.offsetWidth)?document.body.scrollWidth:document.body.offsetWidth;
div.style.height = (document.body.scrollHeight> document.body.offsetHeight)?document.body.scrollHeight:document.body.offsetHeight;