No. MP4 is a container. H.264, H.265, and even MPEG-4 Part 2 can live inside it. People say “MP4 quality” when they mean a typical H.264 encode. The extension never told you the bitrate or the profile.

Why did a remux fail in the browser?

The streams were not legal in the destination box, or this wasm build could not copy that codec. Videozify then needs a re-encode path, which some presets already use. A failed copy is not an upload. The file never left the tab.

Does changing container reduce quality?

A true remux does not. A re-encode does, a little, because it writes a new H.264 file. Read the format page. MKV to MP4 aims at copy. VOB to MP4 re-encodes MPEG-2. Those jobs are not the same.

Why won’t Windows play my iPhone MOV?

Often HEVC without the right extension pack, or a QuickTime-only atom. MOV to MP4 with an H.264 encode is the compatibility fix. The original MOV can stay the archive. A renamed file with the same HEVC inside will still fail on a PC that never learned HEVC.

Are subtitles in the container or the codec?

Usually the container. Videozify’s browser jobs do not promise a subtitle menu. Soft subs can vanish on a remux or re-encode. Burned-in captions are picture and will survive a video encode, looking as baked as they were.

Guide

Video containers vs codecs: why MKV, MP4, and MOV aren’t what you think

A container is a box with a file extension. A codec is how the picture and sound inside are compressed. MP4, MKV, and MOV can hold the same H.264 video. Changing the box can be a remux that copies streams. Changing the codec is a new encode and a generation of loss.

When an MKV already holds H.264 and AAC, MKV to MP4 can copy those streams into an MP4 wrapper without touching the picture. That is the difference this guide is about. If the insides are VP9 or MPEG-2, Videozify re-encodes instead of pretending a remux worked.

What is a video container?

A container is a file format that holds one or more streams plus metadata: duration, language labels, chapters, sometimes subtitles. MP4, MKV, MOV, WebM, AVI, and WMV are containers people name as if they were codecs. They are not.

Players care about both. A phone that “plays MP4” may still reject an MP4 full of AV1. A desktop that opens MKV may refuse a MOV with ProRes. The extension is a hint, not a guarantee.

What is a codec?

A codec is the compression method for a stream. H.264 (AVC) and H.265 (HEVC) are video codecs. VP8 and VP9 show up in WebM. MPEG-2 shows up in VOB and some MPG files. AAC and MP3 are audio codecs that live inside those boxes.

Re-encoding means decoding that codec and encoding another, or the same one again. That is how Videozify writes a silent MP4 from WebM, or an H.264 file from a DVD VOB. It costs time and a generation.

Why can an MKV become an MP4 without looking worse?

If the MKV’s video is already H.264 and the audio is AAC, copying streams into MP4 is a remux. Quality is unchanged because nobody decoded the picture. Videozify’s MKV to MP4 page is that job when the insides allow it.

If copy fails — odd codecs, odd pixel formats — the honest path is a re-encode. Shipping a “copy” control that dies on half of real MKVs would be a fake button. The page tells you which path the preset takes.

Why is MOV different from MP4 on an iPhone?

MOV is Apple’s QuickTime container. Modern iPhone clips are often HEVC (H.265) in MOV, or H.264 in MOV. Windows players that “want MP4” are asking for a box they know. Sometimes the codec inside is already friendly and a remux is enough. Sometimes HEVC needs a re-encode to H.264 for an old laptop.

Videozify’s MOV to MP4 page exists because that Apple-to-everywhere hop is a real weekly job. It is not because MOV pixels are magically sharper than MP4 pixels. They might be the same H.264 either way.

Where does WebM fit?

WebM is a container, usually around VP8 or VP9 video and Opus or Vorbis audio. Browsers like it. Editors and some phones do not. WebM to MP4 is a compatibility encode, not a remux into a random MP4 with VP9 still inside — that combination fails in too many players.

MP4 to WebM is the opposite hop for a page that demanded WebM. Expect a re-encode. Videozify’s MP4 to WebM page uses VP8 plus Opus in this wasm build. VP9 in ffmpeg.wasm did not finish reliably, so the page does not advertise VP9.

How should you choose a Videozify page?

Pick the pair that names the box you have and the box you need. If you only needed the sound, extract audio or a *-to-mp3 page. If you needed a smaller file at the same box, compress. If you needed fewer pixels, resize. Those are not container tricks. They are new encodes on purpose.

Do not chain remux → re-encode → remux hoping for a free quality win. One honest encode from the best file you still have is the method.

What about AVI, WMV, and old camcorder files?

AVI is a container that often holds MPEG-4 Part 2, MJPEG, or ancient DivX. Those insides are not H.264. Videozify’s AVI to MP4 page re-encodes to H.264 so a phone will play it. That is a generation, not a sticker change.

WMV is a Windows Media container around VC-1 or WMA. WMV to MP4 is a compatibility encode. MTS from a camcorder is usually MPEG-2 or H.264 in an MPEG transport stream. MTS to MP4 exists because editors and phones prefer an MP4 box.

VOB from a DVD is MPEG-2 in a DVD container. VOB to MP4 always re-encodes here. Copying MPEG-2 into an MP4 is legal in some desktop ffmpeg command lines and still fails in half the players people actually own. The page chooses the encode that plays.

Do chapters and extra audio tracks survive?

Often not in this wasm build. Videozify’s jobs target a playable H.264 MP4 or a soundtrack MP3, not a museum archive of every MKV attachment. Forced subtitles, fonts, and secondary commentary tracks can vanish.

If you needed the director’s commentary, keep the MKV. If you needed the film to open on a TV’s USB port, the remux or re-encode is the right loss of extras. Those are different jobs. The format page names the one it actually runs.

Frequently asked questions

Is MP4 a codec?: No. MP4 is a container. H.264, H.265, and even MPEG-4 Part 2 can live inside it. People say “MP4 quality” when they mean a typical H.264 encode. The extension never told you the bitrate or the profile.
Why did a remux fail in the browser?: The streams were not legal in the destination box, or this wasm build could not copy that codec. Videozify then needs a re-encode path, which some presets already use. A failed copy is not an upload. The file never left the tab.
Does changing container reduce quality?: A true remux does not. A re-encode does, a little, because it writes a new H.264 file. Read the format page. MKV to MP4 aims at copy. VOB to MP4 re-encodes MPEG-2. Those jobs are not the same.
Why won’t Windows play my iPhone MOV?: Often HEVC without the right extension pack, or a QuickTime-only atom. MOV to MP4 with an H.264 encode is the compatibility fix. The original MOV can stay the archive. A renamed file with the same HEVC inside will still fail on a PC that never learned HEVC.
Are subtitles in the container or the codec?: Usually the container. Videozify’s browser jobs do not promise a subtitle menu. Soft subs can vanish on a remux or re-encode. Burned-in captions are picture and will survive a video encode, looking as baked as they were.