I agree the name 'poster' is not great.
The need comes up very simply: video, when paused, normally shows the current frame. Video starts off paused when it's loaded, and the initial time is 0 (start). Regrettably, it's fashionable to 'fade from black' at the beginning of some kinds of content, and others (notably trailers) start with something that is completely unhelpful (e.g. the green rating screen):
(I hope you are an appropriate audience, or better, can distinguish -- unlike me -- an appropriate audience from an inappropriate one.)
Yet other movies start with a short advert that is not, in fact, indicative of the subject of the video (and adverts are very often faded from black).
To alleviate these cases where the first frame is remarkably unhelpful, we make it possible for the page author to provide something that is, in fact, indicative. (QuickTime movie files solved this with a field in the header that indicated the 'poster time', so for QT Movies it really was a 'representative frame of the video').
So, from this need, one can see it's not a slick poster with text that is almost-invisible-grey and vertically stretched (or horizontally condensed, depending on whether you are a half-empty or half-full type of person),
(and where did *that* fashion come from?) it's just something to help the visually-enabled viewer to well, see something while the movie is paused at the beginning. It gives them a clue what the movie is about or might contain.
And *that* is what we need to do for accessibility: give *everyone* a clue about what the movie is about or might contain. Not give them a clue about what the clue we give to other users might contain; who needs clues about clues?
David Singer
Multimedia and Software Standards, Apple Inc.
