It's an HDR video and it's the OS that controls how it looks. If your display is HDR (OLED and Mini LED Apple displays) it will brighten just the video pixels. If your display is not HDR, it will do as you describe (across the whole OS, not just the webpage). Apple calls this EDR I believe.
The context here is that the BBC recently produced a documentary about Modi that has been controversial in India. Hence the attack on Churchill and the explicit mention of the BBC.