I think it's been hurting heads for many years now. https://www.google.com/#q=html5+video+stretch finds you may have to find other video players or just bite the bullet and make the video exactly what you want in the first place.
James, I supplied the search so you could see the priors. Check the latest HTML5 video standards. I read this:
Quote from the HTML5 spec for <video> tags:
Video content should be rendered inside the element's playback area such that the video content is shown centered in the playback area at the largest possible size that fits completely within it, with the video content's aspect ratio being preserved. Thus, if the aspect ratio of the playback area does not match the aspect ratio of the video, the video will be shown letterboxed or pillarboxed. Areas of the element's playback area that do not contain the video represent nothing.