From HTML WG Wiki
No Poster Alt
Written by Philip Jägenstedt with contributions from David Singer and Eric Carlson (add your name here if you edit the wiki page).
<video poster> is conceptually part of the media resource itself. Any short text alternative should apply to the resource as a whole, not just the poster image.
<video> element shows its current frame when paused. When first opened, the current time is zero, and therefore this is the first frame. The first true video frame is often unhelpful: a fade from black, the green MPAA rating, the roaring lion, etc. To alleviate this, we provide the possibility of indicating an image which is more indicative of the content, and indeed, is ideally drawn from the content: the
<video poster>. As such it is part of the content, and to the sighted user indistinguishable from the content. Given that sighted users won't know (or, if they do, care) that they're looking at the poster image, it would be confusing and inconsistent to have a short text alternative specifically for the poster image for non-sighted users.
If the first visible frame is semantically significant – whether or not it is supplied as a poster file, or is, in fact, the first actual video frame – it should be described as part of the description of the video element itself.
No spec change is required.
If the name is confusing, it could possibly be changed; however, 'poster' is the term commonly used in the industry, and changing to another term may be more confusing than helpful.
We could perhaps improve the documentation, but it's not required; add after "The poster attribute gives the address of an image file that the user agent can show while no video data is available" the phrase (which documents current behavior in all browsers) ", or until playback starts, if desired."
Discussion can move on to short and long text alternatives for a media resource as a whole, the provision of transcripts, and other accessibility aspects of the media resource as a whole.
Conformance Classes Changes