Fullscreen YouTube playback enabling G-Sync
When watching YouTube in full screen G-Sync appears to be (likely unintentionally) enabled, this can cause videos to 'flicker' especially while paused, a side effect of the browser not redrawing frequently enough to keep colours stable. This can be easily verified by turning on the G-Sync Indicator (in Nvidia Control Panel top menu, Display -> G-Sync Indicator).
One way to 'fix' this to set the GSYNC - Application State to 'Disallow' within Nvidia Inspector for the Vivaldi exe, however Vivaldi developers may want to investigate a better solution as allowing video to playback using G-Sync would improve playback smoothness for all video frame rates if done correctly. Alternatively, alerting Nvidia to this issue may get them to fix it.
I know this topic is kind of old, but I'm having similar problems. I have G-Sync enabled for my desktop, mostly because of borderless window fullscreen games.
I'm getting very distinct G-Sync flickering on my Asus monitor when running any sort of HTML5 video in fullscreen, like Youtube. The kind of flickering you notice when a game is running with less than 30 fps, like on loading screens. It's especially noticeable when pausing the video.
This effect does not happen with Chrome or Firefox. I've tried disabling GPU acceleration, but that does nothing.
It seems to have to do with the way how often the image gets redrawn. Less than 30 leads to this flickering issue.