But that has nothing to do with the 'poster' attribute on the 'video' element! Despite its name, this attribute is not intended for picture of a poster for the movie. It is intended for a preview frame to show in the player before the movie starts playing - the spec suggests one of the first non-blank frames of the video.
It's not like they suddenly want to disallow alt text on movie posters on the web.
Nobody wants to deny users anything. It is not really a question of pro or con accessibility despite the editorializing in the HN headline. Its a question of content versus presentation.
The first-frame preview is an artifact of the visual presentation of the video player and not relevant for non-sighted users, just as the colors used in css is not relevant for non-sighted users.
The actual content is the video, and if you want to make that accessible you should follow the recommendations in the spec and "provide alternative media streams and/or to embed accessibility aids (such as caption or subtitle tracks, audio description tracks, or sign-language overlays) into their media streams".
Watching poster frame is not a goal, it's only one of the ways to recognize whether video is worth watching.
There may be better ways for users with impaired vision to decide whether they want to watch a video.
For sighted users poster is useful, because visual recognition is faster than reading of descriptions.
However, when user has to read some description, then reading of description of one of the frames won't necessarily be faster or more informative than reading title or description of the video itself.
Poster frames of videos on the web usually aren't the hollywood kind of posters. They're usually a random frame and/or logo of the site.
Also, time spent on describing poster could be spent on describing the video itself, which might give better indication whether video is worth watching.
