Eric says in bug 25267 that we shouldn’t start buffering a video if a poster frame is specified. You can see by loading <http://bdash.net.nz/files/video-with-poster-image.html> that we do load the video even when a poster frame is specified.
From bug 25267:
> Comment #8 From Eric Carlson 2009-12-21 12:46:51 PST (-) [reply]
> My comment in about using the poster above is no longer correct. The spec now
> says that the poster should be displayed only "When no video data is
> available". The presence of the 'poster' attribute does not change the loading
> behavior, so if an element has both 'src' and a 'poster' attributes, the poster
> is only shown until the first frame of video data is available.