Youtube player controls will work consistently when player is focused:

• Left / Right (5sec backwards / forwards), Up / Down (volume up / down), Home (restart video), End (skip to end)
• 0-9 (jump to 0%-90%), C (switch closed captions/subtitles), +/- (change cc font size): unaffected

Player is focused right after pageload, no need to click in player frame:

• Click outside player frame to get standard page controls: Up, Down, Left, Right (scroll)
• Press Esc to focus player, Shift-Esc to cycle search box, video list, like buttons / comments.
• Space pauses video on the whole webpage except textboxes.

Global shortcuts, changed:

• Esc (exit fullscreen): additionally focuses player when not in fullscreen
• Space (play/pause): works on whole page, not only when player is focused

Global shortcuts, unaffected:

• F (fullscreen), M (mute)
• K (play/pause)
• J / L (10sec backwards / forwards)
• , (previous frame), . (next frame)
• Shift-, (speed -0.25), Shift-. (speed +0.25)
• Shift-N (next video), Shift-P (previous video)
• / (focus searchfield, only in classic ui)
• ENTER (push the focused button)

Fixes the following awkward behaviour of Youtube player controls after clicking them (making the individual control focused):

• after clicking the mute button Space toggles mute instead of pausing
• after clicking the progressbar Up/Down will jump 10 sec in the video instead of changing the volume
• after clicking the volume slider Left/Right will change the volume instead of jumping the video.
• after clicking subtitle button Space will toggle subtitles
• after clicking settings button Space will toggle settings
• after clicking fullscreen button Space will toggle fullscreen

Sources of Youtube shortcut behaviour:

https://shortcutworld.com/Youtube-Player/win/Youtube-Player_Shortcuts
https://www.hongkiat.com/blog/useful-youtube-keyboard-shortcuts-to-know/
https://support.google.com/youtube/answer/7631406?hl=en
2010: flash player? https://www.makeuseof.com/tag/comprehensive-guide-youtube-player-keyboard-shortcuts/