Keyboard shortcut to decrease playback speed on YouTube doesn't work with Chrome (on german keyboard layout?)
I have exactly the same problem and found a solution:


Go to Preferences &gt; Keyboards &gt; Input Sources
(Systemeinstellungen &gt; Tastatur &gt; Eingabequellen) and add another
keyboard layout, e.g. "English".
Activate the keyboard input in the menu bar (checkbox below called
"Eingabequellen in der Menüleiste anzeigen" in German).
Now you can switch the keyboard layout to English and then
Shift-press "," or "." on YouTube in order to slow down or speed up.



  Background information:
  
  On US/UK keyboards the &lt; and &gt; symbols are on two different
  physical keys next to the m key and both of them have to be
  shift-pressed.
  
  On a German keyboard the &lt; and &gt; symbols are both on the same
  physical(!) key (right next to the left shift key).
  
  Probably this is the cause problem, so the &lt; does not get detected,
  because YouTube thinks both functions (speed up, speed down) need the
  shift qualifier pressed simultaneously.