Note that IRC is a standardised protocol (RFC1459).
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Many web sites implement proprietary chat "applications" using e.g. Java or ActiveX, to avoid the need for the user to install an IRC client. These are often highly browser-specific.
ChatZilla is an IRC client written as a FireFox add-on.
The main drawbacks of IRC are the cryptic commands and the geeky culture. Like usenet, only more so. The "alt" hierarchy may have been created for Anarchists, Lunatics and Terrorists, but nowadays most of them are to be found on IRC.