quoted from the article

Comet application fight lag by avoiding HTTP and TCP/IP set-up and tear-down and a single connection is re-used. But the big kicker for AJAX is polling latency which Comet avoids. The big takeaway: transfer only the necessary data, exactly when it’s most relevant.

There are two implementation techniques: Long-polling where you reconnect after every datagram. This is simple to implement with XmlHTTPRequests. Another method is to use multi-part XmlHTTPRequests. This works differently on IE and Firefox and doesn’t work on Safari. No known system does this portably today.