Thanks for your answers!
The broadcast I want to display is my own and it’s definitly not encoded in a 4:3 format. I also changed the standard thumbnail into a custum 16:9 one in the video manager. When I look at my videos, the image Twitch uses is also the 320x180 sized but I don’t know from where they take it.
Thats of course an idea, but I want to make the website responsive, so to fix the image size doesn’t work. Nevertheless, when I try it, I have still the upper black border shown in the div.
So this doesn’t work. I need to get the 16:9 format, just like Twitch does when displaying my past broadcasts.