Except when it doesn't. I use Safari iOS (because there's basically no choice since only their engine is allowed) and I regularly come across webms that don't load or that decode atrociously. It would not be that bad if only they would allow other browsers to ship their own engine.
But why are you blaming Safari for that instead of the site choosing to serve up that particular format rather than something more well supported?
Well, one important part of the web developers job is to /not/ expect their production to work on every browsers. It’s even an important singularity of this job.
I see too much webdevs folks around me who conveniently work with Google Chrome on their 32 inch screen on a $3k+ workstation.
The web is meant to work everywhere and fail gracefully when it can’t. And this rule applies also if you despise a browser.
Specifically not a standard?
https://w3c.github.io/mse-byte-stream-format-webm/ is better in that it's now an "Editor's Draft" but still says "This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress." which strongly implies to me that this is far from a "web standard", no?