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Q. How to play YouTube videos at the highest quality and with subtitles
You can play YouTube videos at 1080p or even at 4K resolution with SMPlayer. You only need to make a small change in the SMPlayer configuration. In Preferences -> Network select mpv + youtube-dl in the option Support for video sites. Also be sure mpv is selected as multimedia engine in Preferences -> General.
This modification also makes the YouTube subtitles available in SMPlayer.
Note: Linux users must install youtube-dl.