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Download Opera browser with:
Opera
Opera GX
Edge
SlimJet
And in case I misunderstood you it's D3D11 in Opera because I was testing whether it was working again and it's implied that "Default" is D3D11 and it's D3D9 in GX because that still works without black screens (although possibly impaired performance?) All other browsers haven't had their settings fiddled with.
@leocg
opera://media-internals
Also, I forgot to mention earlier that in Opera the Default for Choose ANGLE graphics backend is listed as "Default [Undefined]" which is not how it is on other browsers. Don't know if that's an issue too at all.
Both Vivaldi and Opera (regular, haven't looked at GX yet) are using hardware acceleration. The boxes / sliders are ticked / on. Vivaldi is a brand new install in fact. I can post screenshot if you don't believe me! 
Does it make a difference if you run the ://gpu output before and after watching a video? Do the black screens suggest a crash?
ignore-gpu-blocklist didn't change anything BTW. It's not a particularly exotic card anyway I don't think.
Another new install: Brave (no black screens, report made after watching YouTube video.)