seems like the codecs=‘something’ is REALLY nitpicky. managed to get a test video to play with:

<video> 
    <source src="test.mp4" type='video/mp4; codecs="avc1.4d401f"'>
</video>

I made the video by encoding some random clip with ffmpeg -i random_video.mp4 -c:v libaom-av1 -crf 30 test.mp4 (seems to work just as well with libsvtav1)

As for how are you supposed to know the “4d401f”? beats me, found it here: https://caraya.github.io/av1-video-demo/

edit: https://developer.mozilla.org/en-US/docs/Web/Media/Formats/codecs_parameter#av1 does say that the codec string should look a bit different, but… I dunno, not a video-understanding-webmonke.

edit2: and now I realize that since it works with the codecs=avc1 - it’s the older av1 variant? Not really what you were asking. Whoopsiedaisy.