I think you missed the point of my comment : as long as JavaScript is enabled, the website can bypass the "Video decoding API" and just show the media in a <canvas> tag.

Of course, we might be able to find a middle ground between disabling JS entirely and auto-playing all media, but it may be very hard, and that "middle ground" might be changing as website developers find new way to deliver auto-playing media that bypasses the browser's restrictions.