TW2 doesn't seem to look at the wsgi SCRIPT_NAME parameter when generating links to CSS. This means that if you mount a WSGI app at a non-root URL, those links stop working.
I see that the TW2 middleware has a script_name configuration parameter, but this means that the middleware has to be configured differently when the application is mounted in a different place. The rest of my TurboGears2 application has needed no extra configuration when changing the mount point (as long as I'm careful to use tg.url to create URLs).
I don't know what the right way to fix this is - presumably always prepending SCRIPT_NAME has the potential to break things for people who aren't expecting it. Perhaps there could be a use_wsgi_script_name boolean config parameter?