WebM

WebM is Matroska with a much shorter list of what may go inside. Where an MKV accepts practically any codec, a WebM accepts three video codecs and two audio codecs, all of them royalty-free. That restriction is the entire point: a web browser vendor implementing WebM knows exactly what it must decode and owes nobody a licence fee for doing it.

It is the format underneath an enormous amount of what you watch in a web browser without noticing. It is also what most download tools hand you when they pull video off a page, which is how a .webm ends up in a Downloads folder in the first place.

Because it is Matroska underneath, converting WebM to MP4 has the same shape as converting MKV — with one difference that matters. MP4 does not accept VP9 or Vorbis, so the streams cannot simply be re-labelled. Something has to be genuinely re-encoded.

A web browser is an application that brings its own media pipeline. When Safari plays a WebM in a page, it is Safari doing the work, using code Apple wrote for Safari. That capability does not become a system capability: the Files app, QuickTime Player and the Photos preview all use the operating system's own media framework, which has no Matroska demuxer — and therefore no WebM support either, since WebM *is* Matroska.

The result is a genuinely odd experience. You watch a clip in a page, tap Save, and the file you just watched cannot be opened by the device you watched it on. Nothing is broken. Two different pieces of software, two different sets of capabilities.

This is the clearest illustration of a rule worth holding onto: on iOS, what a file can do depends entirely on which app is asked. An app with its own decoders is not limited by what the system knows.