Back in June, Sony teased the user interface for the PlayStation 5 during their press conference in which they eventually revealed the console's design. It was simply the PlayStation button (which is located in the middle of each controller) in the center of a dark screen which featured particles floating about. If you can't recall what I'm describing; it's pictured below.
This appears to be the startup screen which will greet us each and every time we turn on the upcoming next-generation console. However, that's as far as the tease went — the event then went back to showcasing various PlayStation 5 games.
This startup screen is worth keeping in mind, however, as the PlayStation 5's bootup sequence appears to have leaked online (via /r/PS5).
PS5 UI Startup Leak (Video) from r/PS5
This video shows someone, with a black DualSense controller in hand, turning on the PlayStation 5 and promptly signing into their profile. Of course, we cannot verify the legitimacy of this video. However, the black controller is somewhat notable.
Not only is the black controller notable because fans have been begging Sony to offer a black version of the PlayStation 5 and the DualSense controller but apparently the PlayStaion 5 devkit include a black controller, which is to say that this anonymous person in the video may well have somehow gotten their hands on a devkit and therefore the bootup would be legitimate.
Regardless, Sony are yet to reveal the PlayStation 5 bootup sequence and user interface so this is the best we've got until they do. There's only just over a month to go until the PlayStation 5 launches on November 19th.