Usefully, the up/down keys don't change the volume unless the player is also focused.
What's incredibly sad is that j/k/l were agreed upon to rewind/pause/forward the video, and these generally work unless you're in a textbox... but dedicated keys were not similarly approved to control the volume.
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My current winner for Absolutely Favorite Piece Of YouTube-Related Keyboard Functionality©®℠ is...
YouTube Music (https://music.youtube.com/)... which uses a different set of fundamental keyboard shortcuts to YouTube. [;]=pause, [j]=next, [k]=prev, [l]=forward, [h]=rewind.
There are two disasters here.
The first is that <YouTube/pause> == <YouTube Music/rewind to beginning>. "Nooooo! I was 2 hours?? and how many seconds??? into this video... >.<". Naturally, in this situation, I ReWoUnD ThE ViDeO To ThE BeGiNnInG MySeLf, so YouTube's "last play position" indicator can't help me here. YAYYY.
The second disaster is that it's obvious a Vim user clearly argued successfully for Music to use Vi-style hjkl, but both the key choices and how they are presented in the UI could not be less intuitive, regardless of whether the user also uses Vim or not.
I copied above exactly how the UI (in the [?] popup) lists the sequence. Specifically, the whole thing is shifted left by 1 position, wrapping [h] around to the right. Fixing that to show the sequence how a Vim user would expect, it looks like: [h]=rewind, [j]=next, [k]=prev, [l]=forward.
But then, any Vim user is going to be utterly confused by this sequence, not only because of the mild readjustment of assigning up/down to prev/next and left/right to seeking - but because Vim "up" [j] is assigned to "next", and Vim "down" [k] is assigned to "prev".
Upside down playlist much?!?!
I no longer use YouTube Music, solely because of the number of long videos I have rewound and lost my position in.
I'm also incidentally trying to passively commit hjkl to memory (you know, the whole spaced repetition thing), and all the above was just too confusing.