I have tried that, but HTML5 isn't universal on Firefox and Opera. If the video doesn't come in WebM format, it uses the default player.
Two people on Steam just told me they also have this, which is perplexing because I spent an hour Googling various combinations of "YouTube" "offset" "shifted" "video" "support" etc. within recent times, pushing off the crap results. In that, I found no other reports, so I thought this was a more isolated problem. Is this just part of all the screwiness YouTube has had lately?