Video formats and codecs: MP4, MKV, H.264, H.265 and AV1

MP4 and MKV are containers: wrappers that carry video, audio, subtitle and chapter tracks plus the timing data that keeps them in sync. H.264, H.265 and AV1 are codecs, the compression applied to the video track inside the wrapper. The extension tells you which wrapper you have and almost nothing about whether the file will play, because playback fails on the codec, and that depends on whether your chip has a hardware decoder for it.

Two registries make the distinction concrete

MP4 is ISO/IEC 14496-14, third edition January 2020, and an instance of the ISO Base Media File Format, ISO/IEC 14496-12, whose eighth edition was published in April 2026. MOV, 3GP, HEIF, AVIF and CMAF are all ISOBMFF dialects: same box structure, different brand. Matroska has been an IETF Standards Track document since October 2024, RFC 9559, built on EBML (RFC 8794), a binary XML syntax that anyone can extend. WebM is a constrained subset of Matroska built for the web.

The codec identifier lists prove the separation. MP4 names codecs with four-character codes registered at MP4RA: avc1 and avc3 for H.264, hvc1 and hev1 for HEVC, av01 for AV1, vp09 for VP9, vvc1 and vvi1 for H.266/VVC. Matroska uses strings, and its list is far longer: V_AV1, V_MPEG4/ISO/AVC, V_MPEGH/ISO/HEVC, V_MPEGI/ISO/VVC, V_VP9, V_PRORES, V_AVS3 and more. Identical bitstreams, two naming schemes, two registries.

Subtitles are the real reason to keep an MKV

Matroska's subtitle codec IDs include S_TEXT/UTF8 (SRT), S_TEXT/ASS, S_TEXT/SSA, S_TEXT/WEBVTT, S_HDMV/PGS for Blu-ray bitmap subs and S_VOBSUB for DVD bitmap subs. MP4 timed text is standardized by ISO/IEC 14496-30:2018, which gives two sample entries in practice: wvtt for WebVTT and stpp for TTML/IMSC, the pair CMAF, DASH and HLS use. There is no standard MP4 carriage of SRT or ASS/SSA. Matroska also carries attachments natively, which is how a release ships the fonts its ASS track needs. MP4 has none, so a styled ASS track with embedded fonts cannot survive the conversion. That is how subtitles disappear.

If a player refuses your MKV, remux instead of re-encoding. Rewrapping H.264 video and AAC audio into MP4 with ffmpeg -i in.mkv -c copy out.mp4 copies both bitstreams byte for byte in seconds. Re-encoding costs quality and hours. It matters most on the web: MDN lists MP4 and WebM as supported by every browser and does not list Matroska at all, so an MKV holding the same H.264 stream YouTube decodes all day will not play in a <video> element. The browser cannot parse the wrapper.

The state of the codecs, mid-2026

Hardware decode is the compatibility line, not the browser version

Apple is the cleanest illustration. Safari 17 and later decodes AV1 only where the silicon can, which MDN records as M3 MacBooks and later, iPhone 15 Pro, and iPhone 16 and later. There is no software fallback on Apple platforms. A 2021 M1 MacBook Pro running the newest Safari cannot play an AV1 file; a much cheaper recent iPad can. Netflix dates the shift: AV1 hardware support arrived in Apple's M3 and A17 Pro chips in 2023.

Decode and encode are separate engines. Apple's MacBook Pro specs list hardware H.264, HEVC, ProRes and AV1 decode for M5, M5 Pro and M5 Max, with no AV1 encode on any of them. NVIDIA has decoded AV1 since Ampere and the RTX 30 series in 2020, but AV1 encode only arrived two years later with the RTX 40 series and its eighth-generation NVENC, claimed at 40 percent better efficiency than H.264. Blackwell and the RTX 50 series brought 4:2:2 chroma, not first AV1 encode. AMD decodes AV1 from RDNA 2 and the RX 6000 family, with Navi 24 as the documented exception, and encodes from RDNA 3. Intel reached each new decoder first: Lunar Lake and Core Ultra 200V shipped H.266/VVC decode up to 8K60 in September 2024, and Meteor Lake before it had none. The cheapest discrete cards with AV1 encode are still Intel's, at launch MSRPs of $249 for the Arc B580 and $219 for the B570.

Installed base should govern your encoding decisions. ScientiaMobile's WURFL data put AV1 hardware decode at 9.76 percent of smartphones in Q2 2024, with the A17 Pro in the iPhone 15 Pro and Pro Max the single largest contributor at 3.72 percent. Big screens are far ahead: 88 percent of large-screen devices submitted for Netflix certification from 2021 to 2025 supported AV1. The Android 16 compatibility definition now requires handsets to both decode and encode AV1, at [5.3/H-0-6] and [5.2/H-0-3].

Software decode does work. dav1d carries roughly 40 percent of Netflix browser playback. It costs power: Meta measured an open-source AV1 encoder drawing 14 percent more power than H.264 on a Pixel 8, then switched to a low-complexity encoder that returned to parity, and now ships mid-range phones that receive AV1 but send H.264.

Two specific failures to check first. On Windows, Chrome plays HEVC from version 107 only where the operating system supplies the decoder, and Chromium Edge needs the paid HEVC Video Extensions add-on from the Microsoft Store, so "H.265 won't play on my new PC" is usually a licensing problem in technical clothing. Firefox now enables HEVC by default on Windows from 134, macOS from 136, and Linux and Android from 137. The second is hvc1 versus hev1: two registered MP4 codes for the same HEVC bitstream, differing only in whether parameter sets live in the sample entry alone or also in-band. Practitioner consensus, rather than any Apple document, holds that Apple's players accept hvc1 and reject hev1, giving an incompatibility warning or audio-only playback. The fix needs no re-encode: ffmpeg -i in.mp4 -c copy -tag:v hvc1 out.mp4.

What the bitrate savings actually are

Netflix's production numbers beat any clip-set benchmark. AV1 sessions use one-third less bandwidth than both AVC and HEVC, average 4.3 VMAF points above AVC and 0.9 above HEVC, and show 45 percent fewer buffering interruptions. AV1 powers about 30 percent of all Netflix viewing. Meta's September 2025 white paper puts AV1 above 70 percent of watch time across its apps, and its engineers measured at least a 20 percent bitrate reduction against H.264 on low-end and mid-range devices. The gap is widest at low bitrates: at a hard 100 kbps cap, Meta found H.264 noticeably blurry where AV1 stayed clear.

Content matters more than the codec pairing suggests. AV1's film grain synthesis, which strips grain before encoding and re-synthesizes it parametrically in the decoder, gave Netflix a 36 percent average bitrate cut at 1080p and above but only around 10 percent below 1080p, and took one grainy title from 8,274 kbps to 2,804 kbps. Treat the familiar claim that HEVC halves H.264 with suspicion: a peer-reviewed HDR measurement found 14.16 to 38.86 percent bitrate saving, mean 25.08 percent, and 28.46 percent on average at fixed PSNR. Any comparison that omits the encoder preset is worthless: SVT-AV1 alone spans 14 presets, from research-grade 0 to real-time 13.

Royalty-free now needs a footnote

AOMedia's Patent License 1.0 grants a perpetual, worldwide, no-charge, royalty-free license over the claims necessary to the AV1 spec, and terminates your licenses if you sue an implementer. The limit is structural: the grant reaches AOMedia members carrying licensing obligations and the companies that ship implementations, so a patent holder who did neither promised nothing. Sisvel's AV1 pool prices that gap at EUR 0.32 per consumer display device and EUR 0.11 per non-display device, dropping to EUR 0.24 and EUR 0.08 at the full compliance rate. Its listed owners include Dolby, ETRI, NTT, Orange and Toshiba, the licence excludes chipsets, and Sisvel claims to have licensed about half of AV1 finished products. Access Advance opened a rival route in January 2025 with its Video Distribution Patent pool, which covers HEVC, VVC, VP9 and AV1 under one rate.

Litigation followed. Dolby sued Snap on 24 March 2026 in the District of Delaware, case 1:26-cv-00317, plus two Brazilian filings the day before, seeking an AV1 injunction on US patent 10,404,272 and arguing that it never participated in AV1's development and so made no royalty-free or FRAND commitment. US district court AV1 suits reached an all-time high in 2025, with InterDigital, Dolby and Nokia driving the spike. The older codecs got more expensive too: Via LA replaced the $100,000 flat annual cap on AVC subscription and pay-per-view internet video with tiers reaching $4.5 million per year for new licensees from 2026, grandfathering anyone holding an active license at the end of 2025.

For anything you publish, follow MDN's ordering: AV1 with Opus in WebM as the primary rendition, H.264 with AAC in MP4 as the fallback, which plays everywhere with no hardware question. Add HEVC only if your audience is Apple-heavy, and tag it hvc1. Keep your archive in MKV, the only one of the two containers that preserves ASS subtitles, embedded fonts, chapters and multiple audio tracks. AV2 is worth watching but not shipping: a peer-reviewed measurement puts it 29.81 percent better than AV1 on PSNR-YUV and 33.79 percent on VMAF under random access, but no consumer chip decodes it and no browser plays it. Chips&Media finished an AV2 decoder IP block and licensed it to a North American customer on 25 June 2026.