People are way overreacting to this. HTML already lets arbitrary applications register themselves as protocol handlers at the OS level, so for instance spotify

People are way overreacting to this. HTML already lets arbitrary applications register themselves as protocol handlers at the OS level, so for instance spotify:// links get handled by the correct binary. All this does is let a website register itself as the handler of a particular protocol. This has obvious uses for instance letting mailto: be handled by Google Mail if the user wants it to be. I'm not sure why HTML5 has to have a whitelist of protocols that allow websites to register as handlers - it seems like an overhead.