This extension will adjust the pitch of HTML5 Videos to compensate for instruments that are slightly out of tune and are more difficult to tune (like a piano). As a result, you can play that old janky piano along with YouTube videos, without needing to get it tuned. There are also a couple other features - changing playback speed and pitch unrelated to an instrument.
Updates planned for the future: integrate transposition option into instrument feature so they work at the same time, at the moment, you can either transpose based on a number of semitones OR based on the instrument, but not both.
Note: there have been a couple issues with this extension messing with the CSS of websites. The issues have mostly been fixed but if you notice your favourite website now looks weird, consider disabling this while you're not actively using it.