http://www.w3.org/mid/B6CB855C5769484F862F4FB2CCFA50F402D545A7@VHAISHMSGJ2.vha.med.va.gov

The Video Element

There does not appear to be any provision for providing alternative text
for the Posterframe of the video.  I understand why this would be
something that is difficult to require, but it should be possible, and
even recommended.  After all, there might be a video whose title is "A
Day At The Beach".  Knowing whether the posterframe contains the image
of a child throwing a ball into the surf for their dog, a surfer heading
into the waves, or an attractive couple holding hands, conveys very
different information about what that video might contain.

[split out from bug 13590]