I would expect my dedicated media player to handle them, and not a browser. Maybe i'm just too logical. :p I wouldn't press them to control video playback in my browser, i really only use them for music playback while working/developing, and if they stop working in that scenario, thats a big regression for me.
Anyhow, thats why i said it should handle them when its actually playing something. If you play music and watch a video at the same time, something funny is going on anyway.
If you just have MC running to serve DLNA or whatever, but not playing anything locally, it might as well leave the keys for someone else to handle.