Simon Willison points at Charles Millers’ Rules of Argument. This excellent article teaches how to avoid online arguments:
Rule one is scarily simple. You will never change anyone’s mind on a matter of opinion. Someone going into an argument believing one thing, and coming out the other side not believing it is a freak occurrence ranking somewhere alongside virgin birth and victorious English sporting teams. People change their minds gradually, and if anything a prolonged argument only serves to back someone into a corner, huddling closer to the security blanket of what they believe.
…Once you have stated your case, there’s no point re-stating it. Going over the same ground repeatedly will damage your case: nobody likes reading the same interminable debate over and over again. Similarly, if people read what you have to say, understand it, but continue to disagree anyway, there’s nothing more you can do unless you suddenly come up with a totally new argument. The only productive thing you can add is if people clearly don’t understand what you?re saying, and you need to clarify.
…Sometimes, you’ll ignore all these rules, and get into a month-long argument about RDF with a fundamentalist gun-nut emacs-user. What then?
The ideal attitude to project during any argument is one of calm disinterest.
I find that I subconsciously already deduced Charles’ rules for several years; I can’t even remember getting into a heated online argument. I suppose I can thank my faithful readers for already being calmly disinterested – hopefully not just disinterested 
.
Nevertheless, it seems that there’s been a seasonal increase in the number of arguments I read about. Even such an amiable fellow as the AccordionGuy recently had to post a comment etiquette notice. Shelley Powers posted on blog commenters’ hostility on the same day. So did John Walkenbach, who put his finger on the main issue: a weblog (or nearly any other publicly accessible page) isn’t a democracy. The owner decides what to put on it, what to cut, which comments to allow. Anyone who disagrees is welcome to publish his views elsewhere. If you read the comments in the above links, you’ll see that, while a majority of commenters agree with that, by no means all do.
The situation on the Internet contrasts to what many people are used to regarding other media. If a newspaper or TV station publishes something you disagree with, they’re often obliged to allow you equal time or space to disagree; after all, very few people can open their own newspaper or TV station to do so. The Internet and the explosion of free weblog providers changed all that; anyone with the resources to read something also has resources to publicly disagree with it.
