I imagine it has something to do with the keyboard shortcuts for video player controls, which I'm going to list because I think they're neat and was very excited when I discovered them.
space - play/pause
j - skip back
k - play/pause
l - skip forward
0-9 - skip to the n/10th point of the video
m - mute
f - fullscreen
t - "theater mode" which is the bigger-but-not-full-screen video size
I'm sure there are others, but those are the ones I use often enough to remember. I like to use K to play/pause because I never end up accidentally scrolling down into the comments section.
There is indeed more. And there is one hotkey to rule them all, pressing "?" which shows all hotkeys.
"?" is one hotkey I keep pressing in a lot of random contexts, just to see what it does. In many applications it shows all the hotkeys, or if you add some modifier like shift or alt.
Pressing "?" in Gmail shows a large help window with tons of shortcuts (you need to have keyboard shortcuts enabled under Settings > General for this to work).
F1 is now occupied by the native applications help page. Web applications/pages then need to adopt a different one. "?" kind of also make the UX a bit more obvious, honestly. F1 is based around tradition, "?" is based on the idea that you have no idea.
space - play/pause
j - skip back
k - play/pause
l - skip forward
0-9 - skip to the n/10th point of the video
m - mute
f - fullscreen
t - "theater mode" which is the bigger-but-not-full-screen video size
I'm sure there are others, but those are the ones I use often enough to remember. I like to use K to play/pause because I never end up accidentally scrolling down into the comments section.