If I try to play the movie in the URL it goes in slow motion (less than 20% of the intended speed) and the sound is horrific. The user can experience this as a failure of the browser. I suggest that the browser should detect if the movie can't run correctly, and do an attempt to resize the movie.
I don't really think there's much we can do. The problem won't be helped by resizing the movie.
Firefox should at least warn you if it's going to try to do something that's going to lag out your computer, and give the option to tell it never to do it in future. Is there somewhere is the <video> spec that allows a website to feature detect a browser's ability to play a specific video?
We now skip the decode up to the next keyframe if the decode isn't keeping up with the playback, and we skip drawing frames if we're falling behind.