I think I'm leaning towards removing the existing rules and footnotes about portability and adding only the following guidelines:

> Don't use a bin/sh shebang unless your script will be tested on X, Y, Z platforms
> Don't use non-POSIX features with a bin/sh shebang

I don't feel that targeting POSIX should even be a stated preference since it should be up to the developer if portability is more important than convenience -- and even one who chooses portability will have a difficult time actually achieving it.