I'm not sure how well known this is. I just discovered it accidentally and flipped my noodle. Totally brought cheer into my dark, lonely abyss of a night.
You can either do this when your video is buffering (when you see the little circle in the centre of the video), or you can tap the left or right arrow on your keyboard to briefly make this possible. You may need to click on the video first to make sure it's what your commands are being applied to - don't worry about whether you've paused it because this works whether the video is playing or not, but just be sure the video is the active part of the page.
So, either when the buffering circle is visible, or immediately after pressing the left or right arrow, tap the up or down arrow on your keyboard. The directional arrows on your keyboard are now your controls. Enjoy, don't hit the sides, and try not to eat your tail!