It's almost 2025 and it seems like Media Queries for the Video element are back! After reading this article by Scott Jehl (thanks Scott for all your initiative in bringing this feature back), I ran Walter Ebert's test page it in my Safari, Firefox and Chrome, and it worked!!

It can look as simple as this...

<video>
    <source src="small-video.mp4" type="video/mp4" media="(max-width:768px)">
    <source src="big-video.mp4" type="video/mp4">
</video>