ISO Base Media File Format (MP4)
MP4 is the dominant video container on the web and in MSE players. Box hierarchy, codec vs container distinction, and fragmenting (fMP4 for HLS/DASH) are essential for media engineering.
What It Defines
Box-based container format ('ISO BMFF'): nested boxes/atoms (ftyp, moov, mdat, trak, mdia…) carrying one or more media tracks (H.264/H.265 video, AAC audio, subtitles, metadata). Used by `.mp4`, `.m4a`, `.m4v`, `.mov`, and HEIF/AVIF. MIME types: `video/mp4`, `audio/mp4`.
Canonical (Normative)
Convenient (Practical)
Joint ISO/IEC technical committee (JTC 1) publishes many of the binary file formats the web depends on: JPEG, PDF, MP4/ISOBMFF, ZIP (ISO/IEC 21320), and office document containers. Specs are often available via national body mirrors; some are freely readable online.
Related Specs
Video players (YouTube, Netflix, Twitch) use MSE for adaptive streaming. Required if you build a custom media player or do in-browser video processing.
WebM is the default output of browser MediaRecorder and a common delivery format for open web video. EBML element structure differs from MP4 boxes.
H.264 remains the most widely supported video codec. Distinguish codec (H.264 bitstream) from container (MP4/WebM) when debugging playback, transcoding, and MSE SourceBuffer codec strings.
AVIF is the next-generation web image format — `<picture>` fallbacks, CDN support, and browser adoption make it worth knowing for asset pipelines and performance work.