HTML5 video with SVG poster image as data URI
With SVG as a poster image you can set the correct proportions before the video has loaded.
<video preload="none" playsinline controls
poster="data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%2216%22%20height%3D%229%22%2F%3E">
<source src="video.webm" type='video/webm; codecs="vp9, opus"'>
<source src="video.mp4" type="video/mp4">
</video>
Video source: Sintel, the Durian Open Movie Project