From HTML WG Wiki
added fact that @alt can also be sued on the area and input type=img elements
- If we allow authors to specify any image as the placeholder image, we have given the author a means to add additional content/information to the page above and beyond the actual video, using a graphical (visual) means to do so. We have no means however of ensuring that the placeholder image is semantically linked to the video.
When an author adds a video to a web page, the intent is to share that (video) information with the end user; when that same author goes to the effort of creating and posting a separate image file to be used in association with that video (as a placeholder graphic), a means of conveying intent and meaning of that image file should also exist, independent of the video file: there have been 2 creative visual efforts undertaken by the author, and each requires a means of textual representation. (This is further confirmed by the fact that both the video and the placeholder image are referenced by URI strings fetching 2 independent binary assets on the server.) The current proposed guidance for determining @alt text is dependent on author intent and context:
"Is alt attribute content a replacement for an image? The answer to this question depends on the content of the image and the context the image is being used in" (http://dev.w3.org/html5/alt-techniques/)