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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.)
OK, but give me some time. I sort of wanted an excuse to install more browsers anyway so this is as good as any.
Interestingly Edge (latest version, don't know if the version numbers have anything to do with Chromium builds. Doubt it.) is using D3D11 if Default is that as stated in the blurb ("most computers").
Anyway, initiate post dump...
Opera
Not quite the end of this. I'm not 100% sure but I think I'm right in saying performance is not quite as good as in Edge (the only comparison I've done yet). That might have something to do with the number of tabs open (O- like 20 and E- 2) but I'm looking at the Stats for Nerds on YouTube and the frames dropped is over 100 on Opera and only around 30 or so on Edge. Again, this might not mean anything as it might be dependant on how long the video's been playing and such but I think my eyes are seeing a difference? It is hard to know whether it's just my bias because I was having problems before. Anyway, just wanted to mention that.
@leocg Thanks for your reply; setting it to D3D9 fixed it.
So switching to D3D11+ is new then. Which is what causes the black screens for me, both of them (D3D11 & D3D11on12). OpenGL doesn't create black screens but the performance is poor. Which is odd because I have a 1050 (not Ti). Yes I have a shitty Athlon II to go with it but I wouldn't have thought it would have had such an impact on the graphics card.
I wonder what the problem is. A bug in the code or some other symptom of my aging hardware? Anyway, things seem OK now and I can stick with Opera which is nice. As the little guy, it's really come from nowhere to be a better contender now. Hopefully D3D9 support won't be dropped any time soon...
Thanks again. Oh, and is this a known problem for other people too?
Your link doesn't mention that people who aren't staff can't access the support site but if that's the case then I'll summarise-
Opera used to be fine. Since the v80 update, when I attempt to watch a video e.g. YouTube I get a black screen appear for a second (although I can still see the taskbar), I can hear the video play a bit, the screen comes back only for another black screen and possibly a third. Then the video will have paused itself. Or shows an error message. Playing again produces another black screen and eventually the video will play but the framerate will be so bad as to make the video unwatchable. Going to another video might produce the same black screens or it might work albeit with the awful framerate. Turning off Hardware Acceleration gets rid of the black screens but I'm still left with bad framerate because everything's going through my CPU rather than the GPU. I have updated my graphics card drivers to the newest version.
I won't be doing the second thing in the list at the link you sent me because I can't be bothered potentially messing everything up and I don't see why I should spend a lot of time diagnosing something that was perfectly fine before but has now been changed for some reason. If that is a problem then I'm sorry you feel that way. What I wanted to know was whether anyone else was having the same issue hence my title for this thread. Someone saw fit to change that however. "Videos aren't working" isn't a useful title because they are; they just aren't working well. I'll live with it though. It's not like I can change that title again myself.
Videos still play correctly in Edge. Haven't tested anything else yet.
Windows 10 x64. Nvidia graphics.
I made a video at the site where Opera told me to (that apparently nobody else can access). But there's no way for me to attach it here. I'm not uploading it somewhere else to link to again.
Re: https://opera.atlassian.net/servicedesk/customer/portal/18/GB-4439
This now applies to regular Opera too as my installation version has caught up with GX. I've had absolutely no response at that site apart from a system e-mail so I'm assuming this is where to get an actual reply. Is it simply because my computer is so old that I'm having this problem? I've seen people having trouble with the Linux build with the legality of a codec or something but am I the only one with these video issues on Windows? It's going to mean having to move away from Opera entirely if that is the case going forward with this new version 80. Thanks for any help, even just to tell me everyone knows about it already and that I need to read other topics.