The web app IDE has switches for OSX,Windows, Linux and Xojo cloud. Considering that any compiled web app ends up as a CGI file, why would you need build switches, especially since the author of the app usually wouldnt know the browser target audience.
Even a server has an operating system. Most use Linux/Unix, but perhaps you want to host the cgi in your Mac. I even heard that some users are using IIS (from Microsoft) as server.
The browser or OS the user uses does not matter. You select the target for the OS that will host the app.