I just found out if you type j when watching a YouTube video, it lapses back about 10-20 seconds, depending on the video, I think.
What are the keyboard shortcuts when using YouTube?
It's at the bottom of the Screen reader support page:
Keyboard shortcut Function
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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.
In addition to Zom-B's answer,
Keyboard shortcut Function
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J 10 seconds backwards
K Pause / Play
L 10 seconds forward
In addition to other answers:
Keyboard shortcut Function
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M Mute / Unmute
Keyboard shortcut Function
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F full screen
Keyboard shortcut Function
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< Slower (3 levels)
> Speed up (3 levels)
Keyboard shortcut Function
------------------------------------------------
Pause then Left/Right Framestep
Press ? or shift + ? for a list of keyboard shortcuts (may only be on html5)
In addition to other answers:
Keyboard shortcut Function
------------------------------------------------
Left followed by Up Snake game :o)
Keyboard shortcut Function
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Shift+N Next video
Shift+P Previous video
Jumps to the next or previous video in a playlist. On a single video, Shift+N will play the next suggested video.
Note: These shortcuts will only work when the video player is in focus. To move the focus try pressing the spacebar to pause and then play the video.
Additional References: