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
It provides better visual quality at a lower bitrate than the older H.264 (AVC) codec. According to technical discussions on Reddit and GitHub
Supports up to 60fps (High Frame Rate/HFR), offering smoother playback for action-heavy content. 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.
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