Holy crap, that's really helpful. Thank you!

But from the user perspective... how was I ever supposed to guess that?

FYI the exact wording depends on the browser, but all HTML5 video elements will have that context menu option. And other options like "Copy URL", "Loop", etc. depending on the browser.

Of course some websites like youtube override the context menu event to prevent that from showing.

While it is hardly a discoverable feature, you can override custom context menus (at least in Firefox) by holding SHIFT and clicking.

Many thanks. I wonder how many other features I'm missing out on..