Tell us your opinion

Can you add an option to play audio only without video?
No, because Youtube's Terms of Service forbids this.
How can I search for videos in the playlist?
Just use your browser's search in page feature (its hotkey is usually Ctrl+F) when the videos are played and it will find the video in the list of random videos.
Why no option to force HD?
Because it can only be a hint which Youtube usually ignores if the player is not maximized and if it is then it uses HD anyway if it's available.

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