About // in url before the page name

Hello All,
I am using ASP.Net 3.5. In localhost when i put // in the url before the page name then there comes an error of 400 i.e/ bad request error. When  done this thing on live server then the bad request error does not come but the css applied to that page does not seem to work. I tried by mapping url in web.config but no success. Please help me in getting out of this.
 
Thanks in Advance.   
 
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