Indeed it is possible. There are two principal schemes for active chat, both of which require JavaScript on the client side. On scheme relies on doing a periodic refresh from a frame to a server CGI that responds with the most recent messages. Posting a new message happens from a different frame.

The second scheme relies on streaming a sequence of JavaScript commands into a hidden frame. The commands will append to the message stream in a visible frame. Posting a new message happens from a different frame.

To make either of these schemes work, the server side has to do a certain about a client "capability detection", so that the server can know what flavor of frames and JavaScript to use. If you go either route, you'll end up with templates that have conditional chunks of Netscape or IE-flavored JavaScript.

merlyn has a column that shows how to implement the former scheme. For details on the latter scheme, see this paper, which talks about the scheme from a Java-centric viewpoint.