Tested with Stylus for UserCSS and Violentmonkey for userscripts.
Lower the volume of premiering videos, and restore the volume when the countdown finishes. Does not account for playback rate changes.
Amplify videos to equal loudness using the peak loudness value from YouTube. Not recommended for use in Firefox until this bug is fixed.
Set a video to play back in only the left or right channels.
Select captions only from a list of preferred languages, otherwise disable them.
Highlight and sticky live chat comments from moderators and owners.
Force YouTube Live Chat to poll for new messages every second, instead of the default 5 seconds.
Add seconds to live chat timestamps, and add a link to message authors' channels.
Add menu items to force single- or double-column mode regardless of viewport width, useful for forcing live chat below the video.