edit: after clarification by OP this doesn't relate to what they are talking about.

I don't know quite what you are getting at here? Are you suggesting that you can't install plugins to manage different kind of embeds with Safari? As this is just not true: Java, Flash, RealPlayer, QuickTime, Flip4Mac.... these are all plugins built as standard OSX internet plugins. If you want to develop your own go ahead.

I think what you are trying to get at is that you want things that are being natively handled by the browser to be optionally controlled via a plugin. The fragmented state that is what browsers have natively implemented is quite shocking but having everything optionally overridden by plugins would cause even more fragmentation and even more headaches.