The user often does not have a choice because websites that they must use for one reason or another will require these extensions, just like some required Flash. And that's assuming that the extension even works on their system. The extensions are native code IIRC. Ever get a "sorry, your OS isn't supported!" message on a website that you really need to use to get something done? It sucks, a lot. I don't know why the W3C would want to steer themselves in such a direction, it's the antithesis of what the web was supposed to be. The web is supposed to have a well defined set of standards that anyone can implement and use to view the web. This was never true in practice because of things like Flash, but the W3C never endorsed such a thing. Now they are, and that is sad.