Mp4 - 399

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

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

Format is a video-only stream used by YouTube for high-quality content. Key characteristics include: Resolution: 1080p (Full HD). 399 mp4

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

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

(In this example, 399 is the 1080p AV1 video and 140 is the standard M4A audio stream.) Why Choose 399?

It provides better visual quality at a lower bitrate than the older H.264 (AVC) codec. Usually wrapped in an MP4 container, though it

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