It is easier for a user to push a button to install a plugin than to install a new browser, update the profile with bookmarks and so forth, and learn the new interface. We can trivialize these things, but I am sure we also have as many stories of trying to get someone to use Firefox (with noscript!) instead of IE and meeting significant resistance or eye rolling.

A plugin has the disadvantage and the advantage of supporting users' browser choices.

So basically, it looks petty and unprofessional to say on some minority of websites, "This website is developed and tested for" or "This website will only run on". In contrast, I have seen "Click here to upgrade your Flash Player", and moments later, things just work.

Wouldn't it be wonderful if you could have an HTML renderer plugin? And some kind of system where you could -- with some clear and dumbed-down descriptions of the differences/advantages -- choose between alternatives? And get upgrades?