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For 99.99% of the users of Xoops, using CSS or JS files that are external to your site is a waste of time.
The ONLY times I would worry about this is if:
#1, your host is slow and anything offloaded from your host makes a difference.
#2, You are being charged for bandwidth and you are looking to save anywhere you can.
For most of us the time savings would be barely measurable and by no means noticeable. Unless of course you have a very busy site and lots of users at the same time.
It is hard to use externally hosted CSS file unless you are using a standard one that hasn't been modified. You might be able to use something like Google code or something to post it but I have not done this.
If you are using a standard JS library such as Jquery you could use the library from Google or another host. The point of this is that Google is generally much faster than your host and the bandwidth would be used from Google instead of your host.
Realistically, CSS and JS files are cached in your browser and generally never change so the savings really is only for those members that clear their cache all the time.
Rodney