399 Mp4 -

It provides better visual quality at a lower bitrate than the older H.264 (AVC) codec.

According to technical discussions on Reddit and GitHub , 399 is often preferred because: 399 mp4

It is the most modern codec YouTube offers for 1080p, allowing for higher frame rates that older caps (like 30fps) don't support. It provides better visual quality at a lower

Usually wrapped in an MP4 container, though it is technically "video-only" and must be merged with an audio stream (like itag 140) to create a playable video file. How to use it in yt-dlp How to use it in yt-dlp If you

If you want to manually select this format to ensure you get the AV1 1080p version, you would use a command similar to this in your terminal: yt-dlp -f 399+140 [VIDEO_URL] Use code with caution. Copied to clipboard

AV1 ( av01 ), which is a modern, royalty-free codec designed for better compression and quality compared to older formats.

Supports up to 60fps (High Frame Rate/HFR), offering smoother playback for action-heavy content.