It is possible to direct a person from a website to the Mibbit client, and have it automatically go to a certain channel on a certain server. There is currently two implementations of it. The first method implemented was the web handler version of IRC:// urls. The second, and much simpler method, is the channel@server method.
This standard is actually used in links. For instance, the Mozilla developer wiki gives links that might be useful. For Mibbit, it is implemented by placing the URL http://mibbit.com/?server=<SERVER>&channel=<CHANNEL> in the address bar or placing IRC://irc.server.net/channel in as a link. It is simple in and of itself, and while there are other parts, they have yet to be used in a non-testing manner. If the person makes Mibbit their default IRC Client in Firefox, any IRC:// link on the web goes to Mibbit. Chatzilla overwrites this without a few specific preferences, and other clients can be made to open the links, if they support IRC:// links. Thus, the IRC:// method is preferred over others.
This method is not a standard. It is a convenience. To use, it is basically typing http://mibbit.com/#<channel>@<server> with the @server being optional. If No @server is given, the user will go to irc.mibbit.net.