I don’t think I said that OP was wrong (in fact I’m pretty sure I agreed with OP that it was annoying as a fully abled user until I figured out what was going on), but to elaborate on my earlier point just slightly - once a horizontal slider element is in focus it will always use left and right arrow keys to move if they’re pressed. Up and down always works to change the volume because there are no other elements when the video player is selected that are waiting for an input.

If they made the volume slider vertical instead of horizontal it would end up working actually better in this case (because the vertical slider would instead wait for up and down arrows), although that probably wouldn’t look as “nice”, but I’m not a UX dev so it could probably be jazzed up to look nice.