The numbers "399" and "251" aren't part of a codec name. In the context of YouTube and yt-dlp , they are that act like labels for specific stream qualities.
: Specifies the video format as AV1 (AOMedia Video 1), an open-source, royalty-free video coding format. codecs av01008m08 399 opus 251 new
: This refers to the YouTube-specific Format Code (Format ITTAG). When extracting video streams, YouTube assigns static numeric labels to specific resolutions and codec configurations. ITTAG 399 is explicitly reserved for 1080p Full HD video at 30 frames per second encoded natively in AV1 . Understanding Audio Format: opus (251) The numbers "399" and "251" aren't part of a codec name
: This is the YouTube itag for the platform's highest-quality standard audio stream. What Makes itag 251 Special? : This refers to the YouTube-specific Format Code
: YouTube separates the high-quality AV1 video tracks from the Opus audio tracks on its servers.
is an open-source, royalty-free video coding format designed specifically for video transmission over the internet. It is developed by the Alliance for Open Media (AOMedia), a consortium including Google, Amazon, Apple, Netflix, and others. Breaking Down the Identifier av01.0.08M.08 av01 : Identifies the codec as AV1.
However, if a media framework extracts just the relevant parts, it may play fine. For instance, an HTML5 <video> tag with MIME type video/webm; codecs="av01.0.08M.08, opus" and a separate metadata hint 399 kbps will work.