1. Are you using any Cascaded Stile Sheets and/of JavaScript libraries linked to your main web pages?
I'm not, but... I think mod_defalte's hook is after all that is processed, so it's not especially relevant.
2. If yes, how do you turn compression off for those files in case of Netscape-4 originated request?
http://httpd.apache.org/docs-2.0/mod/mod_deflate.html says to use this sort of thing:
# Netscape 4.x has some problems... BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems BrowserMatch ^Mozilla/4\.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
# Don't compress images SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content Header append Vary User-Agent env=!dont-vary
