My site has an HTML main site and a Zen Cart in a subdomain. They share the same interface design. I need to sort out visitors with older browsers so they will continue to get my current design, and pass more modern browsers to a newer one I'm making. For the html main site this is not a problem, a javascript directs traffic to the appropriate files when the visitors arrive. But the separate Zen Cart store will need more changes than just a new CSS file, so I need to be able to change to a different template set depending on which browser is visiting, if you follow.

Does anyone have a grip on this?