Identifying Namespace Declaration

I am using customized namespace for my XSL. I am using libxml to parse.
How to identify that namespace whether its working or not.
In my case, URI defined in the namespace which is not connecting to
that website (i have checked the website in the separate browser). so
what does it mean?
How does XSL works if namespace defined in that particular URI
(http://) goes offline?

Regards
Srinivas