I think people using video_tag or picture_tag will probably be using polyfils or fallbacks.
While we’re on the subject, it would be great to think long-term about supporting picture_tag too, generally it needs a polyfil and a CarrierWave/Paperclip integration, so it may or may not be something to integrate into the Rails core.
I think Rails should take emerging standards (picture_tag and video_tag) seriously as we transition into this new territory.