请教下JS定时弹出的特效怎么写？

Question

先默认不显示，启用计时器20s后显示     document.write('<style type=\"text/css\">.bb{display:none;position: fixed;top:50%;left:50%;margin:-200px 0 0 -226px;*_margin:100px 0 0 -226px;z-index: 9999;_position:absolute;_left:expression(eval(document.documentElement.scrollLeft+0));_top:expression(eval(document.documentElement.scrollTop+0));_left:expression(eval(document.documentElement.scroll_left+100));"}</style>');     //右侧图片      document.write('<div class="aa" style="width:150px;height:470px;background:url(/news_swt/kuang.jpg);position: fixed;top:50px;right:0px;z-index: 9999;_position:absolute;_top:expression(eval(document.documentElement.scrollTop+80));_right:expression(eval(document.documentElement.scrollright+10));"><div style="text-align:right;"><img src="/news_swt/11.gif" style="cursor:pointer" class="gb_left"/></div><a href="/swt/" target="_blank" style="height:459px;display:block;cursor:pointer;" onFocus=this.blur()></a></div>');     //弹窗内容设置
     document.write('<div class="bb" style="width:452px;height:382px;background:url(/news_swt/swt_by.gif);"><div style="text-align:right;padding:7px 19px;"><a class="gb_right" style="cursor:pointer">&nbsp;&nbsp;</a></div><a href="/swt/" target="_blank" style="height:350px;display:block;cursor:pointer;" onFocus=this.blur()></a></div>');     setTimeout(function () {
         $(".bb").show();
     }, 20000); //20s