I'm working on a site for someone, and their forum allows users to put their AIM name on their contact list. I found out that if I add a link using the aim: protocol, the user's AIM client will pop up.
What I'm not sure about is how to handle browsers that don't recognize that protocol. Is there a way to catch that?
Unfortunately, no.
To be exact, if the protocol is handled by a plugin in the browser, the browser user agent string may (usually) let the server know about it. There are also other protocols that are registered by clients in the operating system and the browser my consult the OS if it encounters that protocol.
To complicate matters further, browsers also behave differently with regards to protocols.