AV1 video format offers better quality than Google’s VP9, even at lower bit rates.
And it promises better compression rates than H.265/HEVC and H.264. Even smaller file size without loss of video quality.
In order to add support for AV1 video decoding on YouTube, you simply go to the TestTube page and select "Prefer AV1 for SD" or "Always Prefer AV1" in the AV1 section of the TestTube page. Check this video.
After you have made your selection, you can go to the AV1 test playlist on YouTube to view how AV1 encoded videos are played smoothly on your computer.
To find out if a video uses AV1 or another codec for playback, simply click on "Percent of loaded video" in the Magic Actions for YouTube™ (offered by mycinema.pro) control panel, then check the "Codecs" section.
If you see av01, it means that a video uses the AV1-codec.