Apparently, playing 320x200 15 FPS native HTML5 (Ogg Theora) video in Firefox on FreeBSD takes less than 1% of a CPU core, and that's with X11 compositing enabled (translucent window), but not AIGLX. Nice. However, the "same" video at 640x480 at 30 FPS takes around 10%. It looks like audio is unsynced with video in both cases.
Surprisingly, even though the video drivers are presumably vastly better on Windows XP, playing the bigger video takes almost 80% CPU cycles of a CPU core. I have no idea why.