But then again the use of colors in an image conveys meaning. That does not mean that there should be alt texts for each color channel. The alternative content

But then again the use of colors in an image conveys meaning. That does not mean that there should be alt texts for each color channel. The alternative content should be provided at a level that makes sense if you don't have access to the specific media. The HTML5 spec specifically addresses accessibility for video like this: To make video content accessible to the blind, deaf, and those with other physical or cognitive disabilities, authors are expected to 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. (http://www.whatwg.org/specs/web-apps/current-work/multipage/video.html#attr-video-poster http://www.whatwg.org/specs/web-apps/current-work/multipage/...) This is much more sensible that providing an alt text for the first frame in the video.