399 Mp4 【EASY • Overview】

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

AV1 ( av01 ), which is a modern, royalty-free codec designed for better compression and quality compared to older formats. 399 mp4

(In this example, 399 is the 1080p AV1 video and 140 is the standard M4A audio stream.) Why Choose 399? According to technical discussions on Reddit and GitHub

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 which is a modern