Used to denote the end of a rant (often wittily, or with self-deprecation). Unlike most HTML tags, which are for visual formatting -- not displayed on the webpage itself but interpreted by the browser to alter the appearance of the surrounding text -- </rant> is for mental formatting, altering the connotations of the text preceding it.
A human brain, unlike current web browsers, is capable of inferring the beginning of a rant without HTML assistance; for that reason (and out of sympathy for those who do not plan their rants but get caught up in them as they type) it is permissable to forgo the opening <rant> tag. Similarly, because a human brain is capable of inferring the end of a rant, </rant> is not strictly necessary, and in practice is more often used to demonstrate awareness of the rantlike nature of the text it applies to than to inform the reader of that nature.
(Here on Everything2, there is a twist on usage not often seen elsewhere: </rant> is often added as commentary by someone other than the author, via soft linking. In addition to demonstrating one's own intelligence and verbal acuity to fellow readers, this serves as a method of communicating with the author.)