It’s an issue with Opera, then. I’d report this as a bug at https://meta.discourse.com if it occurs for all users of Opera.
Alternatively, it may be the video hosting service that that specific video is using (unless you can view no videos whatsoever).
This happens only on devforum I can view any video on any other website with HQ.
I use Opera and I can confirm this is happening to me as well.
If you open it in a New Tab, you can download it without having to use Inspect Element.
Oh, yeah xD 
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I’ve personally had issues with Streamable on Chromium-based (but not Chrome) web browsers. There’s something about Streamable that with certain videos, will cause issues with embeds. It doesn’t seem to be so much a Discourse bug as it seems to be related to the per-browser handling of video streams, as I’ve also sometimes seen a similar bug rear its head in Discord, which is Electron-based (and thus uses a Chromium backend.)
As far as I can tell, it’s just down to how the video was encoded before it was uploaded.
