Some types of video require hardware-based graphics acceleration, which isn't supported by the virtualization platform we use. (This can also, rarely, affect some 3D graphics.) We're working to switch to a more robust framework, and in the meantime, here is a non-exhaustive list of video types known to be unstable - for this or other reasons:
• JW Player (with Flash video)
• MJPEG over HTTP
Additionally, the iOS simulators are currently not enabled for video. As there is no audio driver present on the simulators, attempting to play video on these devices may cause Safari to crash.