YouTube Keyboard Shortcuts – Complete Guide

It's at the bottom of the Screen reader support page:

Keyboard shortcut           Function
------------------------------------
O                           To jump to the beginning of the flash player

Spacebar                    Play/Pause when the seek bar is selected.
                            Activate a button if a button has focus

Play/Pause Media Key
on keyboards                Play / Pause

Stop Media Key
on keyboards                Stop

Next Track Media Key
on keyboards                Moves to the next track in a play/list

Left/Right arrow on
the seek bar                Seek backward/forward 5 seconds

Ctrl+left/ctrl+right
arrow on the seek bar       Seek backward/forward 10 seconds

Home/End on the seek bar    Seek to the beginning/last seconds of the video

Up/Down arrow on the
seek bar                    Increase/Decrease volume 5%

Numbers 1 to 9
(not on the numeric pad)
on the seek bar             Seek to the 10% to 90% of the video

Number 0
(not on the numeric pad)
on the seek bar             Seek to the beginning of the video

Indeed it's very hard to find. They ought to make a dedicated page for this.