✳ ASPECT DIAGNOSE
Diagnose: black bars appear on the top/bottom or sides of the video
Dynamic diagnostic state for this problem. Choose a file to run the diagnostic tree; the page rerenders with ranked causes and recommended fixes.
Direct answer
Black bars mean the source aspect ratio does not match the player's frame. Decide whether to crop content or pad the canvas, then re-export a copy. Do not assume every black-bar look is the same; player padding and exported bars behave differently.
Run the diagnostic tree
The tree combines this problem's static cause ranking with the values the diagnostic engine reads from your file.