DTD not accessible

Hi,

I have tried to access http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd  
and if i happen to do this in ie8 i get http 403 back while it works  
in firefox or chrome. I investigated a bit and it seems that this  
happens depending on the  User-Agent header the browser sends. Does  
anyone know about why ie8 gets 403 ?

Thanks for all help in advance.

Best Regards,
Zsolt Fehér