Load your video and press Reverse. Once processing finishes, preview the backwards clip and download it.
Yes, by default the audio track is reversed along with the picture, which usually sounds unnatural — you can mute the audio instead if you only want the reversed visuals.
Reversing requires decoding and re-encoding every frame, so longer or higher-resolution clips take proportionally longer than a simple trim or format change.
Yes, though very long clips need to be held in memory during processing, so extremely long videos work best on a desktop browser with plenty of RAM.