Comment 20 for bug 412647

Yes. From <http://www.whatwg.org/specs/web-apps/current-work/multipage/section-video.html>:

  Content may be provided inside the video element so that older Web browsers,
  which do not support video, can display text to the user informing them of
  how to access the video contents. User agents should not show this fallback
  content to the user.

... which could be a bad idea, given that vendors are free to support any formats. On the other hand, who would actually make videos available in two or more formats? Real use cases would probably be more like <video src="foo.mov">Get Safari</video>.