If you absolutely must have a short, no audio video that will also reliably autoplay on mobile devices - you can host the mp4 video on cloudinary.com and use two separate embeds in Webflow - one for desktop/laptop and one for tablets and down. The cool thing about cloudinary.com is that you can adjust the crop, size, quality and the automatically generated poster image dynamically via settings in the url string. Upload one video, then dynamically resize for multiple device sizes and types. Below is an example of a dummy embed referencing a cloudinary.com hosted video.
Desktop Embed
<video src="http://res.cloudinary.com/demo/video/upload/ar_3,c_crop/v1520954512/example-video.mp4" width="100%" autoplay loop muted playsinline poster="http://res.cloudinary.com/demo/video/upload/ar_3,c_crop/v1520954512/example-video.jpg">
</video>
Mobile Embed
<video src="http://res.cloudinary.com/demo/video/upload/w_0.40/v1520954512/example-video.mp4" width="100%" autoplay loop muted playsinline poster="http://res.cloudinary.com/demo/video/upload/w_0.40/v1520954512/example-video.jpg">
</video>
If you want to stay native Webflow and use the background video element, this very helpful script allows background videos to play on many mobile devices, and you can even overlay a slideshow on top of the background video.