Oh, are we talking about bad YouTube UX? How about the "feature" where the right and left arrows seek the video 5s forward and back, while the up and down arrows increase and decrease the volume? That is, unless the last UI element you've touched was the volume bar, in which case the side arrows will also change the volume, and you'll have to use the mouse to clear the focus away from that volume bar to be able to seek the video again. I still wonder how they managed to break this despite it having had a sane, consistent, defined behavior for probably over a decade before that point.

> That is, unless the last UI element you've touched was the volume bar, in which case the side arrows will also change the volume, and you'll have to use the mouse to clear the focus away from that volume bar to be able to seek the video again

That is a feature (of the browser). The volume bar is selected so it takes up the controls for left/right (this is what a horizontal slider does I suppose). You can also select the volume button and mute/unmute with spacebar (spacebar does the action of the UI element, like click a button). You can tab around the buttons under the video to select options, etc. all with a keyboard. If a control doesn't support an action, it'll be propagated up to the parent, which leads to the jarring feeling that controls are inconsistent (and also the effects, left-right just adjusts the volume, up-down also plays an animation).

It's the usual low quality Google product, but it does make sense why it is so.