setOpcity()

var oWrap = document.getElementById("wrap");
        function setOpacity(node, level){
            node = typeof node === "string" ? document.getElementById(node) : node;
            if(!document.all){
                node.style.opacity = level/100;
            }else{
                node.style.filter = 'alpha(opacity='+ level +')';
            }
        }
        setOpacity(oWrap, 80);

IE用得是滤镜，其他设置opacity属性