> Building a codec into the browser doesn't get you out of having to explain to your parents how to get codecs on their system.
Yes it does; I don't have to explain how to install codecs because my parents don't download videos, because all the videos they want to watch are embedded in web pages (today via Flash, tomorrow HTML5).
Supposing they did want to download a video file for some reason, then ideally the browser's video player would be just as nice as the OS one, and the browser would take over that duty. That hasn't happened yet, but it will, and the end of that road is Chrome OS. The desktop OS with trusted native apps installed on a local filesystem is becoming obsolete. The future is sandboxed apps cached via HTTP.
Or a video that they received via email instead of over the web?
Building a codec into the browser doesn't get you out of having to explain to your parents how to get codecs on their system. Furthermore, once you then do that, and get the codec installed on the system, then you have to explain to them why video doesn't look the same in the browser, or why the controls are different.
All codecs in browsers do is make MORE work for you, not less.