top: expression(((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+100+'px');

Question

top: expression(((e=document.documentElement.scrollTop)?e:document.body.scrollTop)+100+'px');解释一下这句里面的e是指什么？* html, * html body {background-image: url(about:blank);background-attachment: fixed;}还有这个的实现原理是？